"...to think that mains water, power grid and tarmac roads all got rolled out to 'unprofitable' areas of the UK without the big fuss"
The cost was socialised. Urban dwellers subsidised rural ones.
That's very hard to do in a competitive market because if you price high enough to achieve the socialisation all your profitable customers leave for a cheaper supplier who has lower costs because they only cover profitable areas. You end up with just the expensive rural customers who largely can't afford your service anyway because there's no urban dwellers left to keep the average cost down.
The options are to price according to cost and charge more in rural areas, decline to serve expensive areas at all, or press on anyway and go bust. The government subsidy seems the best option unless you want to build a new nationalised not for profit telco that only serves rural areas.