Getting a mortgage can be a nightmare for contractors, especially those who are non- dom (expats). There are often solutions though and it does very much depend on the situation.
If it is a first property then the situation is easier, there are a few lenders out there, though it is often about the mortgage broker and their relationship with the lenders (incredibly).
For second Buy-to-let properties the it gets a bit more tricky, they often want to see a minimum of 6 months in the UK, which does not work for those on the 90 day rule.
For both, however, I would recommend either Mark Hosker (ex RE 9 sqn lad) at Bespoke Finance
Buy to Let Mortgage Broker - Bespoke Finance or Lisa Orme at Keys. Both have a good reputation in the industry and have helped my CP clients get finance.
Third option, is non status finance. This is historically for those who couldn't get a mortgage by other means and has dried up significantly for residential purchase, but has actually grown and become more popular for investment purchases. This has happened with the growth of peer to peer finance and reduction in bank lending. With investments this means they lend on the property not the person (i.e. will only lend on a sound low risk investment), the have first claim on the property - like a normal mortgage. This is not for everyone, as you are limited on the choice of property (unlikely to lend on buying Mums house etc), but for those who are after more hands off it can be a good low risk option.