HI shawty1984
Sorry to read of your fathers passing. Sending upliftment and support.
If your account says you can add Sky Sports UHD for £3.50 for 18 months then it would indeed be tying you into a new 18 months contract. Also you would off course already need to paying for a Sky Sports subscription and the Sky Sports HD add on.
If the account already includes Sky Sports HD in the price you are paying then I suggest speaking to retentions to see if they can offer the UHD add on at £3.50 a month on a 30 day rolling contract which would then tie in with your current 30 day rolling contract.
Since it looks like you are already out of contract (hence the 30 days rolling contract) it might also be a good idea to test the waters by negotiating a new deal with them for all the services you want/require and if acceptable then accept that new deal and new minimum term contract.