Check out www.askthe.police.uk and look up cautions. it actually says "Cautions are covered by rehabilitation of offenders act 1974 and are now spent immediately. This means if youare asked on an application form if you have a caution you can say NO, unless its a conditional caution"
Might be a good idea to check with BASC legal dept, but its there in black and white, but its a police documents so i for one wouldnt be trusting it