Having worked at Heathrow myself, but not in security, but on the aviation side, I would advise you to stay where you are and progress in different departments within the railway. Personally, I left because I took the redundancy package, would I return? No. Heathrow security pay isn't as good as it used to be, but promotion opportunities are frequent. They have a strong union, which is a plus, and the people with whom you will be working are pleasant and diverse. Heathrow security does not provide any benefits to their employees other than free parking for work.