Blimey! So much carp in this thread its untrue.
In a nutshell, when you are a homeowner (not landlord or got lodgers etc) doing anything with your own heating boiler or any gas appliance, and associated parts connected to that appliance, then Manufacturers OWNERS instructions rule. And 'can' override gas regs.
If you do anything (as an owner) not listed in those OWNERS instructions then you become responsible for your actions. End of.
So whether in your mind you are breaking a seal or not, only tinkering with water and not gas, or the electrics, it makes no difference in the eyes of the law!
But as a homeowner (oddly enough), you can fiddle to your hearts content right up to the point something you do harms yourself or others. Then the law takes over, and will only be interested in whether you have followed the manufacturers OWNERS instructions.
And if you have not followed those instructions, then your going to be up the creek without a paddle. Because even if you can prove you are suitably qualified and registered to work on a gas appliance, you would need to explain away how that appliance came to harm a person. And I 'believe' you can be held responsible for up to 7 years after.
Hedge
Nice edit removing your WB boiler, was it just then you realised your own WB boiler has a big rubber 'gas' seal made against the front casing?
And yeah I agree, you would be a moron to take the front cover off a boiler as an unqualified (resit every 5yrs) & unregistered (every 12 months) person.