For a UCI event you must have a BC licence. TLI licence is not recognised except by TLI domestically, and no-one else. So essentially it's no use. The most cost effective way to get a licence is BC silver membership plus a race licence. Simply go on the BC website, all the info is there. If you are entering a base level race in UK it is possible to use a day licence, but you mention a "big event" for which day licences are not accepted. Afraid you just have to pay your money and get the necessary, along with over 100,000 other members (I think over 35,000 have race licences), but consider the insurance and legal support which I hope you may never need but if you do you'll be pleased you joined!