I get the impression that many in the UK want to be able to boast of a strong currency and "therefore" want to keep the GBP. My earliest recollection of an exchange rate for the GBP against USD was when it was worth something like $2.70 and at the time people were lamenting its fall from more like $3 to the Pound. Lots of countries, and not just colonies, ex-colonies, dependencies, etc., effectively kept their national reserves in the UK - it was called being in "the Sterling Zone". Then it happened that the GBP sank dramatically and came very close to parity with the USD and almost all the Sterling Zone members (of course probably not any remaining colonies or dependencies) fled and switched to a different "hard" currency or a "basket" of more than one. The Sterling Zone went out of existence and joined the Empire on the shelves of history.
The Euro is a more recently formed currency. At the time of its issue, the GBP was worth in excess of EUR 1.60. The GBP to EUR rate now stands at a little over EUR 1.25 - the GBP has fallen by around 28% against the EUR in less than 15 years.
Based on its performance over the last 50+ years, those who want to keep the GBP as a national currency because of its strength are either unaware of reality or else deliberately ignore it. On the other hand, those who want to keep it for other reasons will have to factor in its tendency toward weakness.
In the end, the value of anything in its market, currency included, depends on what a would-be buyer perceives its value to be. Among the better known currencies, the GBP is not much of a favourite - but it may well continue to be the world's best in the eyes of those who don't care about the value of "the Pound in their pockets".