I buy 90% of all my hard parts (hard parts = fans, building materials (mylar/panda/ect), ballasts, bulbs, and some myco inoculant on amazon. its 30% cheaper than in a brick and mortar store. you'll save quite a bit of coin that way. not sure if the UK has amazon available.
4 pots will fit under most hoods (all depends on style and hood light spread pattern for optimum light coverage). I run 4x pots under each hood (I run 3x600hps in flower 4.7gallon pots, and have 4-6 more off to the side 2.5 gallon). this is my tester area for new stuff.
don't get overwhelmed, a new grower and even a long term grower gets frustrated from time to time.