I always hate making these life time changing decisions.
I have been in Doha for 5 years now... There is good and bad as in any country. You are the only one that knows if you are ready to get married in advance to come to Qatar. There are advantages and disadvantages...

Advantages: Your fiance's package with the company should get better than the offer as a single male if you get married. He should be offered better housing allowance.
You don't have to worry about getting sponsored by a company in Doha which is good, however if your husband gets fired God Forbid than you will automatically get canceled with him and you will have to leave the country for at least 2 years before you can return. I am not sure if that law has changed or not on that.

Being a British ex pat you could get by with not getting married and living together but you would have to keep it low profile and you will need to find a job that will sponsor you so you can live in the country you also should get housing benefits if your hired as a single and you can ask to get the allowance instead of having the company provide you with housing in advance.

The first year I moved to Qatar it was very hard for me moving from the States to an Arabic Community and it was much more closed in that time than now. It has opened up a lot.. I would say that Qatar is in between Saudi and Dubai... Religious wise it is more strict.
I love shopping in Qatar and there is many nice places to dine with your fiancee''

I think it is a good opportunity and I would accept it you will meet new people and friends and this is something in the long run you can tell your children about.

In the end its your choice! Wish you luck :)

Cheers

Fairy