I'm not volunteering to critique, because wearying experience suggests I know no better than anyone else what makes a reach-out-and-grab-you submission letter.
BUT, I would counsel that, when approaching UK agents, you need to look on whatever you finally arrive at as
just a basis for more personalised letters. As
@AgentPete said in the other, related, string – agents (UK) are a bunch of individuals, and you need to see your task that way. Check their bio, study the website, see if there's an interview anywhere, social media...
Once you are happy with (?resigned to) what you have, look at it as a set of parts, elements of which you can re-use and move around for different letters. Some (few) like to dive straight into the story, which is a more US approach, while others expect a first para to tell them: first novel, genre, wordage, title, author...
On their websites many agencies have a preferred way to set out a letter. If they do, DON'T do anything else. Give them, as far as possible, exactly what they ask for. Plus, of course, trying to make it read well, perhaps hint at the style of the novel-writing – yes, some do suggest that... And, above all, spell the names right. Probably include the job title, too, if it's more than just 'literary agent', e.g. Founding Partner, Vice President, etc...
It is hard work; I won't lie.
