Whatever release mechanism ends up being chosen, it has to be SIMPLE and with few (or no) exceptions.
Something that can be explained in 1 sentence is best.
"The oldest domain registrant gets the .uk"
"The .co.uk owner gets the .uk"
etc.
Both of the statements above are completely clear, and anyone reading them will INSTANTLY understand which party is likely to get a given .uk domain.
As soon as you start massaging the process to shoehorn in a particular group, or muscle out another, you're introducing a focal point for other "aggrieved" parties to use as leverage.
For example, if you say "charities only" then what about charities on generics? Non-profit - but not actual charities - on .org.uk? Charities who own an "older" .org.uk but who don't use it? Government bodies? Educational establishments? People who've played by the written rules, but not the alleged "unspoken" understanding surrounding the use of .org.uk?
That's just the tip of the iceberg. Start adding exceptions, and everyone not covered by the exception will come out of the woodwork to say "hey, why them but not us?"
For me, the ONLY exception that needs to be in place (regardless of release mechanism) is protection for the 2-letter/1-letter domain auction participants. There is very strong logic to support it, so Nominet has the necessary "cover" to treat them differently.
Everything else, well, keep it super-short-n-sweet and you ruffle the least feathers. Because the more feathers you ruffle, the more challenges will be mounted to your proposals, and the less likely they are to go anywhere at all.