Any decent computer accessories shop (and many stationers) can sell you special iron-on transfer paper, which you can print onto with an inkjet printer.
Some people seem to get good results from using those transfers. Others find that they're very fiddly and/or the images don't survive many washes. (It's really important to read the instructions carefully, and to follow them to the letter).
If you'd rather use a professional service, most High Street photo-processing shops can do it for you, as can the photo counters in the big supermarkets. Asda even let you order online, by uploading your photo. (�7.80 for an adult T-shirt, �5.84 for a child's one):
http://www.asda-photo.co.uk/
(Click 'Gifts', on a blue background, near to the top of the page).
Chris