Congrats on the weight loss!

It's not really an issue from the UK Border arrival point of view, particularly if you use e-passport gates: staff are well trained to see how people change over time. If you changed gender or something then maybe. Think about all those 28 year males that go from being weedy teenagers to bearded spectacle wearing hipsters from their photo as 18 year olds. The e-passport picture quality to the UKBF agent is lousy anyway. That's why there is other information like height and age which are more difficult to change. UKBF have seen everything in their time.

The reason for changing would be more the countries you visit, just in case they are more sensitive to this. US and Australia are the two that come to mind, but again they have the same issues locally, and are well trained.

Get a work colleague that you don't know very well to have a look at the passport photo and make a judgement call.