Need opinion on upgrading groupset for my road bike
That's a nice enough bike, but it's not something to upgrade, you bought the wrong bike. Either you want a vintage bike in which case it sounds great, or you just want a road bike, in which case something with Shimano Claris R2000 would be suitable.
Assuming you have 400EX, then this dates from 1990-1992. 600EX would be pre 1988, so it's possible you actually have Exage Sport, which is more contemporary with 600EX.
631 tubing however suggests more 1990s than 1980s
I would suggest a value maybe £200? https://www.facebook.com/Retrobicyclesrecycle/posts/3754668234607817
There is nothing special about those components, though they aren't bad.
I had a look on fleabay and you could buy a generic modern(ish) bike such as a Triban 540 with 2x10 speed 105 and carbon fork for about £300. Or you could buy a new 2 x 8 speed bike such as the RC 120 for £400.
Not much to do with it except buy some new tyres if they've perished. Sell it if you don't like it. Buy a more suitable bike.