Maximum foreign tax credit relief for interest

When I enter an amount of foreign interest the "Rate of tax credit relief allowed: (optional)" menu goes from 0% to 25%, but my UK tax rate on interest is 45% and the foreign tax rate is 37% (USA). The DTA for the USA has no mention of a maximum tax rate for interest. How can I get a FCTR of 37%? Thanks. Also, even though it says "(optional)" it forces me to select a value.

Hi

Under the terms of the double taxation treaty, tax relief is restricted to 15%

Thank you

Thanks for your reply. Can you please point out where the interest tax relief is limited to 15%? I've looked through https://assets.publishing.service.gov.uk/media/5a81972ce5274a2e8ab54ce7/usa-consolidated_-_in_force.pdf and I see that for Dividends Article 10 sets a limit of 5 or 15%, but I can't find a simliar limit for Interest in Article 11 or elsewhere. Is that the right place to be looking?

Hi ftcr,
It is under Article 11, point 5 -  the gross amount of the interest may be taxed at a rate not exceeding the rate prescribed in sub
paragraph b) of paragraph 2 of Article 10 (Dividends) of this Convention. 
Thank you.