Thanks for the mention, but at present New Forest Brushes do not do custom - just a small range of branded hand made brushes using high end hair grades.
Bearing in mind what you say you want to achieve and how much you are prepared to spend, I'd seriously consider this:
http://fidosshavingbrushblog.blogspot.com/2010/02/simpson-duke-3-best.html
As a member of TSR you can get a 10% discount here:
http://www.diamondedgeltd.com/acatalog/Simpsons_shaving_brushes_and_accessories.html (It's the Simpsons Duke 3 in Best.)
You don't need to spend as much to secure similar quality but although the brushes are not technically made in England they are as "British" as anything you are likely to find and an iconic brand.