If you UK property is/was your sole and main residence, the capital gains are not applicable in the UK. This would mean that you actually lived their for some years before moving to CH, but as current market conditions prevented a sale, you rented it out.

I am a little mystified why the UK tax man would say it you need no longer complete a UK tax form, as
a) you still have income in the UK and
b) this is to your advantage. Mortgage interest and other rental expenses like agent's commission, repairs and maintenance can be set against the rental income.

As the UK and Switzerland have a double taxation agreement, any tax due for rental after the deductions in b) above would only be paid once. Either in the UK or here.

Finally, Switzerland has a wealth tax and your UK property should be added to this on your Swiss tax declaration. Wealth tax is low and measure in per mille rather than per cent.

All I can tell you is that this is exactly our situation albeit some 15 years ago. Then the market prevented us selling for 5 years...