I'm only in Littlehampton, but I go to SMD Newhaven for most of my work.
There's a big gap around us, I have used
http://www.performancetuningservices.co.uk/ for a couple of jobs, get the feeling they are very good but not Subaru experts.
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2005 Spec B 6MT - KiDO tuned (~270Bhp)
Custom exhaust - Cosworth filter - Samco Intake - Tints - Bilstein B8/B6 & STI springs - SuperPro ALK - Whiteline positive shift kit, rack mounts, rear adjustable camber bushes, front ARB mounts & strut brace, 20mm rear ARB with AVO mounts, - Hardrace Gearbox and pitch mounts - Prodrive PFF7's - Front Brembos - Kenwood CarPlay - 1/2 carbon grille - STI front splitter