Perhaps my mistake is forgetting that it is genre editors and agents I am aiming at – they already have their heads partly in my cloud. Unfortunately in the UK there seem precious few of the latter, so I am wary of assuming too much familiarity with the genre when approaching others.
But you shouldn't be approaching agents and editors who don't ordinarily represent SF and Fantasy. That would be a big mistake. Agents who have their own websites, or who you find on a list of agents somewhere, should describe which genres they are will look at and represent. If you are submitting directly to publishers, you should know what the SFF imprints are.
Sending queries or proposals where they won't even get a look is one of the major mistakes that new authors make. Yes, there are a limited number of agents who represent Fantasy in the UK. That is just the way it is, and you can't make it better by hoping to convert an agent who wouldn't ordinarily be interested. Besides, if you did, they would be ignorant of the field, not have contacts in the right places, and do a poor job of representing you. It would be better, if and when you exhaust the possibilities in the UK, to start contacting agents and or editors in the US.
Honestly, you do not have to fill your synopsis with explanations of themes, elements, etc. Concentrate on telling your story in a simple and compelling way.
twin souls linked to a magic system for characters symbiotic with other fantasy creatures
You've just summed up the background for a Fantasy novel in
less than one sentence. See, you can do it. You are
not in the same position as a Victorian explorer describing the exotica of Africa, because the flora and fauna of your imaginary world are not what the synopsis is supposed to be about. Stick to what impacts the story directly, as you did in the sentence above, and you'll do fine.
Don't make things harder than they need to be.