Definitely don't wait until the end of the year.
I'm in the process now.
Do you have a tame AME who's UK CAA approved? There are slightly different processes depending on when you last did a UK CAA medical.
Wrestling with Cellma will age you immensely and will probably bump your blood pressure up.
The website isn't super clear.
EASA to CAA
I ended up paying to get the medical records sent over as well as doing a new medical. The new medical was easy (after wrangling Cellma).
Your CAA reference number is your old number without the FCL bit. Eg. 123456A. Hours etc refer to the last UK CAA medical you did.
Medical About £300, fee to transfer medical £109 (I think), fee to transfer lic about £330.
Scan your passport, current EASA licence and medical and get them certified. I used my AME. You can use your chief pilot etc. The CAA's site has the wording required.