Brew2bottle seem to have good value bottles in stock. 24 x 500ml Amber PET Beer Bottles for £9.45 and cheaper still if you buy 48 or 96. Or 15 x 500ml Brew2Bottle Glass Amber Swing Top Beer Bottles for £12.95. And many other types for wine, spirits and beer!
https://brew2bottle.co.uk/collections/bottles?page=1
https://brew2bottle.co.uk/collections/bottles?page=1