I am moving out to Dubai soon and I made my decision based on the increased opportunities for career progression. I work in construction, hence the array of projects also helped and when I heard that my salary would double and I wouldn't even have to share it with the taxman, that more or less sealed the deal!!

My point is that, this salary is extremely low and when you started thinking about a possible move, you must have had at least a few reasons that governed that decision and hence, you need to assess this offer against those criteria that you set when you first started looking for a job. If the offer fails against your own criteria, I would say walk away as it seems FF are out to take advantage of you. I'm not sure where you are from but ultimately, when you move jobs, especially if you have to jump on a plane to do it and move away from friends and family, it should be because there are greater opportunites and maybe a better salary, better working conditions, etc.
Please do not let yourseld be dazzled by bright lights and make a decision which will leave you struggling to make ends meet and having to hand over such a large percentage of you income to your employer. Even the UK taxman is not that greedy!!!

I would advice doing a little bit more research and being a little bit more patient and hopefully you will be rewarded with a much better package.

Best of luck