Personally I’d stick with Colchester, they’re far more versatile if I had to choose between them.
Your only option is to buy the Holbrook and have both
I have the use of a D13 (also in Wales) which spent several years outside under a sheet. After a bit of cleaning up it's an absolute joy to use especially the tool relief slide for screw cutting, and still incredibly accurate. Obviously a far higher quality machine than even a Colchester in good condition.
But it's limited capacity compared to your Colchester.
I think it really boils down to, exactly how sh*gged out is your Colchester?
Well really it's not shagged, it just has a lot of way wear. Nothing is floppy, backlash is there but not a big issue, head bearings are good. In a world where I didn't have any other pressing projects I would attempt a strip and rebuild, but doing that now would just heap more pressure on me, as I guess would be buying this lathe - getting them swapped would be a major removals job..