For safe(ish) use of a generator in an emergency, I would proceed as follows.

Install a second consumer unit for high priority loads such as lighting, central heating, and IT equipment. Depending on size of generator this might or might not include include general purpose 13 amp socket outlets.

Supply from public mains normally, but with a suitable manual changeover switch to permit use of a generator when needed. Mains input into changeover switch from a suitable MCB in the main consumer unit. Generator input from a suitably rated ceeform Appliance inlet located outdoors.

When the mains fails , plug in the generator and start it, then operate the changeover switch.

Safety features.

Be certain to use a ceeform appliance inlet and not a socket outlet for the generator connection, rating to suit size of generator.

Whilst it should be safe to handle the trailing socket connected to the generator whilst live, as a second line of defence, the user should be told to plug it in and then start the engine, not the other war around.

At the generator inlet bond together the generator supply neutral and earth in order that the generator supply is TNCS. (this is arguably a breach of the regulations If such a bond already exists at the generator, but is in my view a price worth paying to ensure that neutral and earth are boded, when the internal wiring of a borrowed or hired generator is unknown)

Ensure that this neutral/earth bond at the generator inlet is also connected to the existing MET and thus to metallic services and earth rods.

For low loading but high priority loads such as IT and communications equipment, install RED 13 amp sockets, on a 6 amp or smaller MCB, any attempted misuse of these sockets for  kettles, heaters and the like will operate this MCB.

A couple of thoughts ...

 Mains input into changeover switch from a suitable MCB in the main consumer unit.

Personally, I'd try to avoid feeding a CU from an MCB - especially one where you want resilience. Discrimination between MCBs on faults is poor to non-existent regardless of their relative rating so a fault on any of your high priority circuits will quite likely cause the feed MCB to de-latch as well, disconnecting all the priority loads, even though grid power remains. If you need to de-rate below the DNO's fuse, then a HBC switchfuse fed from Henley blocks (downsteam of a separate main switch) would be my preference.

At the generator inlet bond together the generator supply neutral and earth in order that the generator supply is TNCS. (this is arguably a breach of the regulations If such a bond already exists at the generator, but is in my view a price worth paying to ensure that neutral and earth are boded, when the internal wiring of a borrowed or hired generator is unknown)

As long as the generator doesn't have its own link that should create a local TN-S system, rather than TN-C-S, and should be entirely to the letter of all the regs (presuming your change-over switch switches N as well as L). You may (should) still have a solid connection to the grid's TN-C-S earth, but as your generator system only has N connected to Earth at a single point only, your part is still TN-S.

If the generator already has it's own link you technically shouldn't introduce a 2nd one, I'd agree. But most small ones don't - or worse have one centre tapped (so the 115V output is earthed too) - which is going to give you bigger problems.

   - Andy.

A couple of thoughts ...

 Mains input into changeover switch from a suitable MCB in the main consumer unit.

Personally, I'd try to avoid feeding a CU from an MCB - especially one where you want resilience. Discrimination between MCBs on faults is poor to non-existent regardless of their relative rating so a fault on any of your high priority circuits will quite likely cause the feed MCB to de-latch as well, disconnecting all the priority loads, even though grid power remains. If you need to de-rate below the DNO's fuse, then a HBC switchfuse fed from Henley blocks (downsteam of a separate main switch) would be my preference.

At the generator inlet bond together the generator supply neutral and earth in order that the generator supply is TNCS. (this is arguably a breach of the regulations If such a bond already exists at the generator, but is in my view a price worth paying to ensure that neutral and earth are boded, when the internal wiring of a borrowed or hired generator is unknown)

As long as the generator doesn't have its own link that should create a local TN-S system, rather than TN-C-S, and should be entirely to the letter of all the regs (presuming your change-over switch switches N as well as L). You may (should) still have a solid connection to the grid's TN-C-S earth, but as your generator system only has N connected to Earth at a single point only, your part is still TN-S.

If the generator already has it's own link you technically shouldn't introduce a 2nd one, I'd agree. But most small ones don't - or worse have one centre tapped (so the 115V output is earthed too) - which is going to give you bigger problems.

   - Andy.