To make 24VDC supply you will need to find a transformer that will output at 17-18 VAC. Since you live in the UK this transformer will have to have an AC input of 220. So look for a transformer with 220VAC in and 17-18VAC out with the amps you want. The formula is a follows
VAC * 1.4 = VDC. So if you find a 17VAC * 1.4 = 23.8VDC or you may find 18VAC * 1.4 = 25.2VDC.
You must connect the transformer to a Full Wave Bridge Rectifier. Get a 250 Volt rated one. Like from
www.mouser.com a BR250
Next you will need a Capasitor also from mouser. The formula for the minimum Cap is: Cap = Amps * 80,000 / VDC. If you find a 18VAC, 6 amp transformer then you will need at minimum
CAP = 6 * 80,000 / 25 = 19,200 microfarad cap.