If in doubt, cheat - Merlin Photo ID (Certainly on Google Play Store, presumably on Apple) is a marvellous tool - free, though you will get prompts to go on Cornell U. courses, etc. Merlin also has a Step by Step ID tool with it (asking for size, main colours, where seen, what doing, etc. and making a few guesses). Even Google Lens might give you a decent result. Oh, and the most wonderful thing about Merlin is that you can set it to listen to bird song and it will flash up guesses at what it is hearing in real time - warning, this can get a little addictive.

If you prefer books, my current favourite is "Britain's Birds" from WILDGuides, with an RSPB endorsement (even if I hold love for the Hamlyn Guide from 1970 - great art).

In passing, with those additional photos, we can see a white streak over the head, which definitely points towards Coal Tits (and away from the rarer Marsh and Willow Tits).