.UK isn't my focus, but I will say that I'm often approached by people hoping to sell entire portfolios. Usually the portfolios aren't as valuable as their owners think. Plenty are downright worthless. But I've also seen many portfolios of real value – all the way up to portfolios that include individual domains worth high 6 or 7 figures.
Realistically, there just aren't many people who buy portfolios. Period. When the portfolio is quite valuable, then you'll find few buyers with an adequate budget. And when the portfolio's asking price is lower, typically that's because it's made up of many non-premium domains. In that case, buyers won't be interested in 50-90% of what they see. So they'll want to cherry-pick the best domains.
Even rich domainers don't want to spend half a million upwards at a pop. Maybe in the future that will change, as more corporate buyers are beginning to play the game. GoDaddy, for instance, has recently bought 4 portfolios that I'm aware of. And they've gone into the mid 8 figures to do so. But it's not as if the average domainer can walk up to GoDaddy and say, "Buy me out!" In any event, their interest in .UK would be minimal, I think.
I'd say selling whole portfolios is (at worst) an exercise in futility and (at best) would deliver far less money than breaking the domains up and selling them off individually. My advice is not to even try. It's simply a waste of time.