Rather than NatWest and RBS, you can try NatWestInternational and RBSInternatiional. Googling for "offshore morgage" will find other options. As stated, the criteria and charges will be more than for mainstream UK products.

Something to consider if you regularly visit the UK and have a trusted friend or relative who will let you use their address is to get onto the electoral roll and try applying for a mainstream product. It might help if you changed the address for any UK credit card to that address. An independent mortgage adviser should be able to discuss the details and let you know if this is feasable.

Most high street banks have offshore subsidiaries that will accommodate expats. Generally though, the criteria is stricter and the charges higher than for mainstream products.

To qualify for most mainstream products, you would need a UK address and to be on the electoral roll.