I'm able to mix and match because Strings by mail sells half sets and even single strings. I believe they do ship to UK no problem.
I've been mixing for a long time because I rarely find a full set I like. I often like the bass sets of one brand and the trebles of another just as you do. Also the bass set usually loses vitality a bit quicker than the trebles. Personally my favorite combo is Savarez Corum basses with La Bella 2001 MT Trebles. The basses are quite warm, mellow, powerful and are comfortable for the left hand. The La Bella 2001 Trebles I find to be full, warm and colorful yet clear.
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