regarding wiring in fcu

It is hard to be able to say without seeing the installation and being able to do some testing. We should never assume things. Always test to confirm.

The quick answer is... “If you turn off said spur does it turn off both your washing machine as well as your boiler or not?” Basically if it does then yes it controls both items, if it doesn’t turn them both off then no it doesn’t control them both! If wired correctly than when you switch the spur off there will be no power allowed to said units! Otherwise either you have multiple other Spurs that control these units or if you have turned all Spurs off then they have been wired incorrectly and so I’d advise you to seek the help of a qualified electrician.

The FCU could be part of ring circuit, so there should be 2 cables connected to the supply terminals. The boiler should be connected to the load side of the FCU, would usually be smaller cable. Then the socket could be connected as a spur into the supply terminals. So if wired correctly there should be 4 cables. There should not be any boiler control cables just the boiler supply/feed cable. If in doubt best to get electrician to have a look.