The first thing I want to say is this

Not one of the the 25 greatest cricketers of the last 25 years is English. Why is that?

Well, first I should answer a different question - Is that true? I wrote it, and I'm not entirely certain it's true. I'm pretty confident up to about 20, but there's a decent chance an Englishman or two creeps into the Top 25.
You're mad, you're saying. Well, are any English cricketers of the last 25 years greater than these (not in order)?

Warne
Sangakkara
Imran Khan
Hadlee
Marshall
Lara
Pollock
Kallis
Tendulkar
Muralitharan
McGrath
Ponting
Wasim Akram
Lara
Vettori
Ambrose
Gilchrist
Waqar Younis
Waugh S
Kumble
Dravid
Inzamam
Donald
Flower A
Hayden

The answer is no, my friend. Ian Botham would creep into the lower echelons of that list if we could take into account his achievements before 1981 but his stats throughout the 80s are very average, and, anyway, I haven't included Kapil Dev, whose career test stats are pretty much his equal.
All the chaps above dwarf and dominate anyone England has produced in terms of weight of runs/wickets, impact on the game, status, watchability, everything ... and ... and ... I've just realised I didn't include Viv Richards, who, though, like Botham, he was at his best a little earlier, still achieved enough in the late 80s and early 90s to comfortably merit inclusion. Obviously. Because he's one of the greatest cricketers of all time.
Vettori, you say? One of a select band with over 300 wickets and 3000 runs in tests, consistently in the Top 5 one-day bowlers in the world, keeper of the flame for left-arm spinners, one-man bowling attack, distinguished captain, hell yes, Vettori is a greater cricketer than Andrew Flintoff. Chris Cairns is also a greater cricketer than Andrew Flintoff. Mohammad Yousuf is a greater cricketer than Kevin Pietersen. Mark Taylor is a greater cricketer than Michael Vaughan. English players are nowhere. Gooch, Gower and Stewart deserve some consideration, but still, the list does not suffer by their absence. All averaged below 45 (admittedly at a time when averages were generally a little lower). Sangakkara averages 55. And kept wicket. And captains, successfully.
But.
The funny thing is I come to praise English cricket, not to bury it. For, in spite of the absence of these stand-out players, England haa still managed to compete, for the last ten years, at the upper echelons of the game, somewhere between 2 and 5 in the world, capable of series wins against anyone else. This speaks highly of England's team spirit, unity of purpose, ability to perform when the chips are down etc. It's the very opposite of the football team in a sense, where many of the most highly rated players in the world over the last 10 years have been English, and they have managed to be far less than the sum of their parts.
Still, the inability to produce great players is a worry. Without providing definitive answers, I can make a few observations.
1. The two England cricketers of the last 25 years whose achievements will go down in the history books, Hick and Ramprakash, will go down in the history of first-class cricket, but will be mere footnotes in test cricket. Clearly not because they weren't good enough at accumulating runs. The shambles that was England cricket in the early 90s just destroyed them.
2. In the last 15 years, England has had a slew of high-class, highly effective fast medium bowlers - Gough, Caddick, Hoggard, Flintoff, Harmison, Anderson. All have excellent test records, all reached 200 test wickets (well, Jimmy will soon), averaged around 30, but none got anyone near 25, or got above 250 wickets (Jimmy, one hopes, will do the latter, but i wouldn't count on it). I would suggest this is a positive in a way - it suggests strong competition meaning you'll be dropped if your level slips (as happened savagely to Hoggard), the ability to hunt in packs, to share the load - but still, for England to win series, they did have to perform together. Very rarely has one bowler won a series for England.
3. The failure of any batsman to really stand out has less of the positive about it. All these players in the low 40s, it's just weird that hardly anyone has pushed beyond that. It suggests an inability to step up and say "this is my responsibility, I'm the star here" that being a great batsman needs - batting is an almost entirely self-involved action. A great test batsman is better off forgetting entirely about the concept of team for the time he is at the wicket. Pietersen is the only England batsman who has even threatened to stand out since Gooch, and even he has been sucked back into the mid-40s mire in the last couple of years.
4. Maybe it's just coincidence. Hobbs, Hammond, Barrington, Hutton, Trueman - true greats of the game, English. Maybe it's just been a funny few years.
5. Statistics-wise, it's reasonable to suggest it's somewhat skewed by pitches - if your home pitches are in Asia, you'll have a higher average than if your home pitch is in England or New Zealand.
6. English kids just aren't bred to be great cricketers - you rarely hear about English child cricket prodigies. No one gets groomed for greatness and put in when still in their teens. Look at how Adil Rashid is being handled. The chances are it will help him being a very good cricketer, but it has removed the possibility of him being a great cricketer.
7. Flintoff ought to have been a GREAT cricketer. Talent-wise. His four magnificent years and his unforgettable moments of glory do undoubtedly make him worth more than the sum of his stats. I lied earlier. He is a greater cricketer than Chris Cairns. Just. But his long, slow apprenticeship (partly his own fault) and his long, slow waning (not his own fault) mean that he is a man with 4000 runs and 200 wickets (both at 30ish) rathern than 7000 runs at 40 and 350 wickets at 25. He should have been.
8. Gower ought to have been GREAT too. He should have been allowed to score 11000 runs at 47 rather than 8000 at 44 (incidentally, i think 80s averages are worth 5 runs more in current statistical terms, i'd say), but English cricket messed him about in a way that just wouldn't have happened in other countries. He'd have just been stuck in the team to bat at Number 4 for 20 years, no questions asked.
9. This probably isn't about to change. Broad may just threaten greatness, but I believe he'll finish with Flintoff-esque stats. KP may yet make 10000 test runs at 50, but i think there's more chance he'll be done in a couple of years. England are on course, though, to be a Top 3 team for a time to come. Would I swap that for one true great, the likes of which I haven't seen in my 25 years of watching England? We'll see.