As above - if you are holding only an Unconditional Firm, thats it. Come Results Day that is where you are going. This is WHY Unis offer U places to those still at school - it locks you in and cuts down all your options to go elsewhere. And this is why is isnt a great idea to accept it.
The only other avenue open to you (now) is to phone your ex-Insurance, and find out if they would be prepared to remake that offer in Extra. If yes, this would mean you would have to withdraw from your now-Firm choice and reapply to the other Uni via Extra.
Or of course, on Results day you can cancel you UCAS application, not go anywhere this year and reapply next year.