Add me to mortgage or do a deed of trust

Pippajane

My partner and I live together (unmarried) in his property. He has a five year fixed rate mortgage which runs out March 2022. Last year we went to port the mortgage as we wanted to move and were rejected by his bank as he had an outstanding £1k debt which showed up in a credit check as a default. This didn't go to CCJ, however it did go to debt recovery and he was paying £50 off a month and so showed up as a default for 6 months. It was cleared in full as soon as it came to light on the credit check but understandably Natwest refused porting and adding to the mortgage and said we would have to wait a further 2 years before they would reconsider further lending to us. The 2 years is up in March 2021.
I sold my property last year and now the money is just sitting there so we would like to explore our options, in particular, can I be added to his mortgage and it not affect my credit rating? My credit rating is very good. Or would we be better off me paying off 10% off the mortgage this year and next to get it down and getting a deed of trust written up instead? 
Also is adding me to the mortgage similar to remortgaging?

My mind boggles with it all, thoughts would be greatly appreciated...
Thanks in advance.

Thrugelmir

Adding you to the mortgage isn't the same as a remortgage. However you will make a joint application which will be underwritten taking into account the lenders criteria at the time. Until the fixed term has expired you'll suffer a penalty to leave the existing product.