score:1
Will it matter if my signature does not fully match what I wrote in my application?
No. Variation is normal.
Upvote:3
To have good identical (or very similar) signatures, you need to sign a lot (really a lot). This is not the case for most of people, so it is normal to have different signatures on different documents (and on credit cards, with slippy surface, it is impossible).
The style remain, and signatures should be made fast and natural. So, if one try to reproduce a signature, it is noticeable (a non natural flow). This was mostly important with old pens.
In any case, immigration officer are trained to distinguish a person from a photo (e.g. children), forget documents, and signatures. Graphology helps: some part of your signature will remain the same. Check the flow: the stroke order in a letters, the style of the letters (and groups of letters), etc. The most noticeable part of a signature are also not so important: the pen make us to change the size, the compactness in each direction, etc. It is difficult to random people to forge a signature.
Signature is not very significant for the travel document: photo and physical characteristics are most important.
But such travel documents, used as IDs with photo, are used to authenticate signatures on other documents, and such "authenticators" are less trained. Car rentals and some hotels tend to make a copy of your travel document, and not just trusting so much the signature, but if you think you need to sign some contract in some foreign country, it will safer to have a similar signature as in your document: it will cause less troubles.
Note: the question was UK specific, but passport are mostly used outside own country, so this answer is generic, about how people tend to use signatures.