UKH

Simple question, is is possible to buy some half ropes, where only one is triple rated for the occasions where a single is better? Adding to this, what length would be sensible? I climb in the Bristol area. 50m is usually sensibe around here for half ropes, but I think a 60m would be needed for a single. Is it a stupid idea to have a 60m tripple and 50m half alongside it? Will it be impossible to end up with ropes that feel at all like eachother if I do this?

I wouldn't get 2 different lengths, it would seem too easy to mess up at some point.

I also wouldn't mix my half and sport ropes, as the sport rope really needs to be a bit thicker as it takes a lot of abuse. These thin sport ropes don't last very long.

I use 60s for everything, you be surprised how often you climb somewhere (or have to back off) and will be glad of the extra length.

I do this. I have a 9.1mm triple and a 8.5mm half. Works fine for me. For any route where a single is OK I just use the triple, and add the half if necessary. I'm sure a pair of matched halfs would be slicker, but with the amount of climbing I do it's not worth the extra cost. No problems abseiling either. I firmly believe that 50m ropes are fine for UK trad, and 60m ropes are just extra hauling at the belay. Getting one 60 and one 50 is indeed a stupid idea. I have tried this and it was a complete PITA.

Have used a Mammut Serenity exactly like this for a long time, indeed it was my main rope. Not ideal if you are using it for working sport routes but great for trad.

IMO, having mismatched lengths can be turn to your advantage, just build your belay out of the longer rope. It gives you 5m of building each belay which is just about cock on. 

"cock on".   Is that a new belay building technique?  Sounds painful.

You can build your belays any way you like. Including your cock is not essential.