There are multiple mainstream lenders - including Nationwide - who do not ask about criminal convictions. They are more concerned about affordability - can you demonstrate sufficient income less expenses to cover the repayments.
The thing to be careful of is Buildings insurance as these will ask the question or will have it somewhere in their T&C's that the premium is based on no unspent convictions. The approach I used was to decline the insurance from the mortgage lender (which is usually sensible anyway from a financial perspective) and then arrange via a specialist insurer from Unlock's list of brokers. Your solicitor will need to see a copy of the policy before completion to be able to tick a box for the mortgage company before releasing funds.
The other option is to use a financial advisor / mortgage broker who you would be prepared to share your conviction information with. They will then use a lender that they know does not ask the question - with a broker the lender also does less due diligence of the borrower as there is an implicit assumption that the broker has done their own KYC/diligence checks on their clients.