UK tax residency certificate and claiming tax relief on foreign interests

[super1]

UK tax residency certificate and claiming tax relief on foreign interests

It's a bit confusing, so would be appreciated if someone could explain me that. If I live in the UK and

1)
I have a UK tax residency certificate in a foreign bank, tax on interests is taken there 10% and then in the UK I can claim tax relief on interests which means 20-10=10% I have to pay 10% tax on foreign interests in the UK

2) if I do not have UK tax residency certificate so the foreign bank takes 15% tax on interests, then in the UK am I allowed to claim tax relief on interests 20-15=5% and pay 5% left in the uk tax on foreign interests or as I dont have that certificate in foreign bank, I can't not claim that 15% in the uk and pay 20%?

thanks

[maths]

No you can't claim for the 15% foreign tax; only the 10% (which I assume is the rate applicable under a double tax treaty).