A while ago I asked my GP to change my title of address from "Mr" to "Ms". It turns out whatever system the NHS (or possibly just my GP's surgery) uses to keep their records absolutely didn't expect that and has no provision to handle it. I bet it's perfectly possible to change from "Ms" to "Mrs" or even from "Mr" to "Fr" or "Dr" but try to go from "Mr" to "Ms" and the whole thing HCFs. So there's now two records for…
I suspect they used to use title as a proxy for gender but this wouldn't be very sensible so maybe it's just there for historical purposes or no particular reason.