Just engines: http://www.justengines.unseen.org/ go to the Monthly section to see the offers on at the moment, or the Catalogue section to see the full line up, including pipes. Always worth ringing them, they know their stuff.

Island Hobbies: http://www.flycapiche.co.uk/ or http://www.islandhobbies.co.uk/4584.html The FlyCapiche site has the prices in the orders section, Island Hobbies has the contact details (both under construction hence the chaos). They are in Jersey, so prices exlcude VAT. May be added by customs if you are unlucky, so best to include it when comparing any prices.

Weston UK: http://www.westonuk.co.uk/ If you want anything from them, give them a ring as they don't reply to emails. Personally, the other two would be better.

Other good sites for UK stuff include:
http://www.inwoodmodels.co.uk/
http://www.alshobbies.com/
http://www.sussex-model-centre.co.uk/