Me and the missus have been working in the UK for a month now. We have been told by one manager that we need to fill in a WRS (Workers Registration Scheme) form and pay the 90 pound fee to keep working. However another manager has told us that, due to the WRS becoming illegal in April (as confirmed by the home office website) that we don't have to.
The WRS is still in effect until the 1st of May, so yes, you need to register.
However, the chances of them prosecuting anyone now for not registering is unlikely, to say the least. As they say - you pays your money and you takes your chances.