Beer online

Not in any particular order though my recommendations are stronger for the top 5 or so: 
https://honestbrew.co.uk/shop/ 
https://shop.hopburnsblack.co.uk 
https://hoptimism.co.uk 
http://www.beerritz.co.uk 
http://www.beergonzo.co.uk 
http://radbeer.com 
http://www.beermerchants.com 
http://www.hophideout.co.uk 
http://www.alesbymail.co.uk - mostly UK stuff but does have some pretty small/new/unheard of breweries 
http://www.beerrevolution.co.uk - mostly just UK stuff 
http://www.beersofeurope.co.uk - mixture of craft and macro stuff from all over the world, also useful for some harder to get country ticks and Belgian/German beers. 
https://www.eebria.com - shipped direct from each brewery so only worth it if you're buying in bulk from each brewery you're interested in. 
http://www.beerhawk.co.uk - ABInBev owned for a couple of years now 

A few others I know of but haven't used yet so can't vouch for them 
http://www.aletavern.com 
https://brewcavern.co.uk 
http://www.leftfieldbeer.co.uk 
https://www.beerheroes.com 
http://www.beautifulbeers.co.uk/ 
https://imperialbeerclub.com - Used to be a monthly subscription service but now also sells singles - only stocks imperial strength beers 

EU sites who (I think) deliver to the UK: 
https://boxbeers.com [Denmark} 
http://www.beerofbelgium.com/en/ [Belgium] 
https://www.beergium.com/en/ [Belgium] 
https://www.belgianbeerz.com/ [Belgium] 
https://beerrepublic.eu/ [Netherlands] - mostly for US/Canadian beers 
https://bierhut.com/ [Netherlands] 
https://www.bier-deluxe.de/ [Germany] 
http://biershop.bierpost.com/de/ [Germany] 
https://ebrowarium.pl/ [Poland]