Let's put the numbers into perspective...
If you have 1,000 .co.uk domains you need to find 3,000 pounds a year in renewal fees approx. (assuming the renewal dates distribute evenly since each renewal is for 2 years)
Of course, you could very well cover that amount from parking income alone, if at least some of the domains get "useful" amounts of traffic.
But it would also only take one sale a year at 3,000 pounds (a very achievable amount for many domains when you look at the domain sales charts) or 10 sales at 300 pounds each to also cover the renewal fees. In other words, for an annual turnover of just 0.1% to 1% of the total portfolio - assuming the names aren't complete junk - you can cover your renewal fees in full. After that, all the income from parking or further sales is gravy.
Under such relatively "easy maintenance" conditions, why should larger domain portfolio holders be tempted to sell for anything less than end-user level offers?
Another difference between the .com and .co.uk markets is the relative ease of closing a transaction. In .com, the entire transaction could be done electronically in under 5 minutes (and at zero additional cost) if both parties are on the ball (Paypal payment, push to purchaser's registrar account) but with Nominet's paper forms the process can take weeks, and it isn't free either.
As such, people may be willing to flip .com names all day for a few tens or hundreds of pounds profit per 5 minutes of effort expended, but .co.uk names require MUCH more effort and hand-holding.
One more consideration is the very different levels of type-in traffic between .com and .co.uk generics, while at the same time the sale-side metrics for .com are very firmly established. A domain like logo.com might get many thousands of typein visitors per day, whereas my logo.co.uk gets somewhere in the region of 70-100 daily visitors. As such, if you can pick up a generic .com "bargain" (e.g. a true commercial, generic name with useful traffic at 3x annual income) you can immediately resell it for 5x-8x annual income or more (sometimes much more) as there are hundreds of eager waiting buyers.
The .co.uk market by contrast moves at a more sedate pace. You don't see the likes of a BuyDomains, a MarchEx or a Name Admin rolling up hundreds of thousands of .co.uk generics into a massive portfolio... nor are there buyers lined up, competing to buy generics at decent revenue multiples.
Finally, from participating in the .co.uk and .com markets for many years now, I've seen a much higher level of "outrage" in .co.uk against paying end-user prices. While the .com world is finally - albeit slowly - shaking off the "domain resellers are cybersquatters" image, there are still regular articles in the .co.uk press that make absolutely no distinction between people running a legitimate business selling generic.co.uk and blatant cybersquatters selling trademark.co.uk.
This combines with a more general reluctance in the UK to help somebody else "profit" and means that many people shopping in .co.uk don't even BEGIN to understand why a seller wouldn't be happy with a 1,000% profit for a strong generic ("hey, it only cost you 6 pounds so 100 pounds is a fantastic return. How DARE you ask for 10,000 pounds for topgeneric.co.uk - cough, splutter, %&&$((# you dirty cybersquatter, you")
The above might be summed up as "In .com, many companies have a good idea of the approximate value of a decent .com domain. In .co.uk, most companies still live in cloud cookoo land when it comes to actual domain values."
