If the gasket has been blowing across the gap between 1&2 and 3&4 the metal could have been eroded away, but only over very many miles. So I doubt this analysis. A warped head is much more likely.
The 9mm head clamping bolts are not good enough nor get tight enough with angle-tightening, try my suggestion in the guide for fitting a mk2 head on a mk1 block - tap out the block threads to 10mm and fit good studs and big nuts. Worked a treat for my 1994 55S which was blowing gaskets regularly, it is still running 3 years later and never uses a drop of water. If you are thinking of another block you have nothing to lose.
Don't worry about the ITV test if you have to change the block. They don't check engine numbers and most testers wouldn't know where they are stamped on. And they are not all totally unreasonable.
I wouldn't ever get a block skimmed, the pistons might protrude too much when refitted and hit the gasket or the head and valves. I don't like skimmed heads either as the deck is thin to start with and skimming makes things worse especially if plenty is removed.
Make sure the 2 locating dowels are short enough - the head should not 'rock' when offered up with no gasket, if it does it may not compress the gasket enough. And quality gaskets MUST be used, there is some junk around.