Some of the suppliers I have used so far for my Buildlog 2.x Laser build.
Not all are actually in the UK, but all are the cheapest I have found so far once P&P/duty has been included.
20x20mm, 20x40mm extrusion and brackets
Motedis - even cheaper than KJN, work out a cutting list that uses 6m lengths to maximise savings, they will cut it and include the offcut for the price of the 6m length.
T nuts M4 or M5 for 20x20mm extrusion
Robotdigg based in China but still the cheapest by far. ($7.50 for 100pcs) need to specify M5 in follow up email or you will end up with M4
Screws, bolts, nuts, washers, studding/all thread, etc
Orbital Fasteners free next day delivery for orders over £20.00 + VAT within van area (greater london, OX, RG, GU, KT, etc post codes), outside these areas free next day delivery on orders over £50.00 + VAT or £7.95 if below.
GT2 Pulleys and belts
Robotdigg (as above) I used 1524mm/762 tooth GT2 belt for Z height adjustment ($2.40), 202mm/101 tooth GT2 belt for Z speed reducer ($0.50), 10 x 20 tooth GT2 pulleys 6.35mm bore $1.85 each, they also sell MXL belts and pulleys.
Makerslide
Amber Spy Glass when in stock
not really too many other choices for this in the UK.
Gantry plates
Blomker Industries $49.00 + $29.00 shipping
V wheels, eccentric spacers and 1/4" spacers
OpenBuilds be careful with the overall weight to avoid excessive shipping costs, 10 x V wheels, 3 x eccentric spacers and 6 x 1/4" aluminium spacers $22.20 + $9.45 P&P.
625 Bearings -
ebay seller
warm-tech from China £1.70 for 10pcs with free P&P.
1000mm of 10x10mm drag chain
ebay seller
lintashop from China £4.08 each + 2.08 P&P (I bought 2 of these).
1000mm of 15x10mm drag chain
ebay seller
quanshop2014 from China £4.25 + free P&P.
Currently looking at
ebay seller
plasticsourceltd for Dibond £39.99 + £17.50 P&P for 1220x2440x3mm sheet in white + free cutting for up to 15 panels.