Whilst £750 is perhaps on the high side, it is roughly in line with what any garage will charge. It's all very well some people quoting £100 for parts, but some of the prices suggested are certainly not going to be genuine good quality parts (a £12 cam belt and an £11 water pump can only be inferior pattern parts), and then there is the cost of a skim if needed, plus labour, for which a bricks and mortar garage is probably going the charge at least 4 hours labour at a minimum of £50 and hour - most will charge more than that; anything up to $100 an hour.
The head gasket has gone, and if it is leaking outwardly, it is very likely that it will leak internally too within a short time.
If it has the original type elastomer SLS gasket, the usual failure is the elastomer bead deteriorating due to excessive heat and becoming unbonded from the metal shim. Magical sealants are designed to seal gaps between metal parts, and even if they work on the loose elastomer bead, the flexibility of the material means it is quite likely to move slightly, which will break the seal and it will leak again.
I would bite the bullet and get a reliable mobile mechanic to replace the gasket - as someone has already said, thay are massively cheaper because they don't have the running costs of a physical garage building (along with the business rates, maintenance, insurance, health safety and fire prevention provision that goes with it). Dave and Kayleigh (
MG Rover Mobile Mechanics) in Derby may be the known reliable mobile K series specialist that are your nearest?