drinking alone on the worst possible night of the week, why do I always do it???

[TitoFan]

back then it was a bottle of Mount Gay, decent stuff but not the best I've had, and £40 a bottle isn't all that much, jesus a bottle of Kraken will set you back £30

Never had Mount Gay, but £40 is roughly equivalent to about $60, and that's ungodly expensive for a good bottle of rum. Caribbean rum goes for far less than that, and you've got some vintage, aged stuff that'll be sure to tickle the palate.

A bottle of captain Morgan is £16 for anything half decent you are paying £25 or more

Don Q Gran Añejo: a bit under $50. Vintage Puerto Rican rum.

I'm not a Captain Morgan fan myself, unless I'm looking for a flavored rum, or for some mixes.
But I can understand why rum would be expensive over there, as it's not produced anywhere near your area.