The blocked cards from manufacturers come with the block installed, they aboslutely are not DIY. The Gigabyte WB for example has liquid metal TIM/a liquid metal composite and a gasket/well to prevent escape. The "Block Installation Guide" you may have seen will relate to possible in/out configurations and the act of putting fittings in etc NOT the actual installation of the block. There is no way they would sell a DIY waterblock card and then offer a warranty on it.
A 3rd party block ALONE is going to cost you upwards of £200. It's not quite as simple as you make out either. They need to design for a reference and custom PCB, manufacture, test and validate. They also need to source cards to design from (Be it purchased or donor). The market is not massive so it all needs to be done with a margin in mind. Hence the ultimate retail cost to the consumer.
If only £350 was £35% of a 5090 cost! Sub £1,000 5090s? Sign me up.
Personally, I don't think saving £100-300 is worth invalidating a warranty on a £2,000 to £2,500 product and having the pain and anxiety of installing a 3rd party block on such an expensive bit of kit. Plenty of people will think differently.
I wont be buying a 5090 with re-sale value at the top of my priority list. I fully understand a waterblocked card has a smaller resale audience but I don't really care.