Hiya James - welcome. A few answers to some of your questions. Viv size: for one adult Leo a 2 foot viv would be perfectly adequate so you could save a bit of cash there. No Leos don't get lonely and often do much better on their own. Tbh I think you would be hard pushed to get the lot for £100 but so that you know the essentials and approx. prices:
Vivarium - £60 or you could at a push use a plastic faunarium £20
Heat mat - £15
Mat thermostat (don't try to do without this no matter what any shop tells you) £20
Thermometer - digital £6
Calcium - £3
Nutrobal - £5
Leo - £20 upwards
Hides, water, food and calcium bowls can be improvised (Margarine tubs, jam jar lids - you get the idea) Paper towel or newspaper for substrate.
This is the most basic but adequate set up I can come up with. You may well be better looking in the Classifieds for a second hand set up. If you do just make sure you disinfect everything thoroughly.
Sorry don't know any of the rep shops in your area.
Good luck and enjoy your new little Leo.