Well when i got a job offer for the Netherlands last year i had to do alot of cost calculations, i found this site quite valuable:
http://www.numbeo.com/cost-of-living/comparison.jsp
Example:
You can easily see what things cost and where you will get nailed (and how much you should at least earn to match your lifestyle in SA). Things that was throwing me off :
1. The actual salary they offer, while translated from euro -> rand was alot, it was alot less in terms of what i can do in europe with it as opposed to doing the exact same thing in SA.
2. Taxes and Pension : You actually need to be a CITIZEN to benefit from alot of these things. In the Netherlands you do get a "tax break" while you're not a citizen, but you don't mysteriously get all the benefits that normal citizens get. Not sure how the UK works with this, but i still had to budget additional for medical aid + pension (same we do here) while paying 30% tax (citizens pay a higher tax rate, more like 40%-50%, not sure).
3. The actual job was/is a step down, with limited promotion/increases ability (Europeans don't seem to be big on annual increases like in SA, usually an increase implies a promotion) , one can see it as a "stepping stone" , but i haven't felt that negative about my current circumstances to want to do that.