Hi Sazid Mohmed
Thank you for your question.
As you have mentioned that you have already created a declaration of trust stating that you are responsible for the mortgage payments and your friend no longer makes contributions to the property, HMRC would expect that as you are the beneficial owner of the mortgage payments, that you would then claim the mortgage interest which is reflected within the Declaration of Trust.
Further guidance on ownership can be found in TSEM9170:
Ownership and income tax