My feeling has changed about them. I used to think they might be a good investment. But when you think about it most have been unregistered for all these years. So they are not that valuable.
There will always be a small market, no doubt between domainers and they may slowly increase in value over the years, as the general value of domain names increase. The best you can hope for is that an end user will appear who just cant live without the 3 letter dot org.uk of their name.
Its up to you what you think the chances of that happening are, but I think you might be waiting for a very long time.
My general advice (without seeing the names) is sell for whatever you can get now, unless its a word with some commercial value - then develop it.
Just think, you are waiting for that 3 letter .org.uk to go up from £10 to £20. Thats 100% profit. But I can reg a commercial .co.uk on a high paying key word, build a minisite on it, make £10 quid over the course of a year and possibly every year that the site is in existence.