I would buy your workhorse bits in 8mm. Most of your routing will probably be covered by a few bits anyway. 8mm are not as common in stores, but easily enough to get online from Weldon, CMT (probably many others too). For small bits and one off, 1/4" will do fine. Anything that can potentially be used in a trim router I would buy in 1/4" so that you can get the trim router at some point and dont need more bits, eg rounder bits, chamfer bits.
Note that 1/4" is 6.35mm but will generally be marked as 1/4". 6mm is a different size which you dont get in the uk. They cant be mixed. 8mm bits are 8mm, they wont be given an imperial equivalent.