Your easiest option is to get two bottles of the same flavor, one in 0 nic and one in 6mg (0.6%) - then you can mix them together in various ratios to get lower nic. A 50/50 mix will get you 3mg (0.3%), 66% 0 nic and 33% 6mg will get you 2mg (0.2%), etc.
You can also get a 6mg bottle and dilute according to the same ratios with plain PG or VG, but this will cause the final flavor to be weaker - possibly *substantially* weaker depending on how much you plan to dilute.
Finally, there's always the DIY route, but it sounds like you're looking to be able to purchase off the shelf... if that's the case, the first option is probably your best bet.
I *think* I saw a US vendor selling some 3mg somewhere, but I can't remember who it was, and the shipping to the UK for one 3mg bottle would probably come out about the same as getting a 6ml bottle and a 0 nic bottle from a UK vendor and mixing it yourself.
(Edited to add): Actually, your *best* option would probably be to seek out any UK vendors that do "made to order" juices and shoot them an email - any made to order vendor should be able to mix any nic level you so desire.