My advice would be to only consider a position if the basic salary on offer is matching or is higher than your current (gross) salary in the UK. Ensure that the living allowance offered covers your accomodation and car expenses. Your employer will usually provide you with medical insurance, so no need to factor it in.
I was living in London prior to moving to Doha, so I can tell you that with the exception of the accomodation (and housemaid) the living cost is pretty similar to the UK.