found some pics of enigmas welding and it looks not bad
I just had a look and it's not half bad! Just re-read my earlier post and it seems a little harsh.. I'd say that the Whyte seems a good bet but isn't custom, the Enigma/Omega is custom but I thought they were very road focused so may not have the feedback from MTBs to ensure their design is spot on, all others are Ti bikes from Taiwan (which in itself is not bad).
If you want truely custom MTB you can't go wrong with a Seven (except with the bank manager!), Sigma sports in Kingston (near London) are the importers and can do a proper measure up to ensure full custom fit rather than their Signature series fit or there are a multitude of other US builders like Black Sheep who could do something.
Personally I just don't think it's something we're set up for over here, especially with our more traditional scene with brazed steel being king. If I want UK I go for steel, if I want Ti I go for US, if I want carbon I go for a head exam!