you have to declare the income and value only for tax rate purpose! It is not taxed in CH, as it is unilateraly exempted form taxation by Swiss tax law. It is nevertheless mandatory to declare it, because, as you are fully subject to taxation in Switzerland, your taxable income will be taxed at the applicable (progressive) rate for your entire (worldwide) income. Therefore if your income is 100 and the rental is 20, you will be taxed on 100, at the rate of 120. Is that clear?

Regarding the capital gain on the sale of the UK house, it will also be taxable in the UK, due to double tax treaty. The profit you will get from the house will be taxable with your wealth (wealth tax) at the end of the year, provided you have not invested it in another real estate abroad in the meantime.