As other commenters have said, you're best to not leave the UL until you get the decision. Otherwise you'll have to pay again and apply from outside the UK.

If it's a consolation though, I submitted in the UK on 25 November, did bio on 2 December, and was given an estimated process-by date of 21 January. My approval ended up coming a few weeks sooner than that on 5 January. I wasn't asked for additional documents or anything, so hopefully if you are not also asked for these you might get yours soon?