According to the UK government website at https://www.gov.uk/maximum-weekly-working-hours the first port of call is ACAS.
Alternatively you can refuse to work on average more than 48 hours per week averaged over 17 weeks (that's what the regulations state...) and your employer can either live with your refusal or they can sack you. At which point you take them to an industrial tribunal for wrongful dismissal.
Effectively your employer can ask you to exceed 48 hours per week until such time as your 17-week average would exceed 48 hours per week... I presume your contract of employment allows this (or does not specifically forbid it) and provides some form of extra remuneration / time off in lieu / states that you have to work as they request without extra remuneration.
Personally if you are in this situation, I'd live with it until it reached the working time directive limit and then refuse to do more than 48 hours per week... and in the interim I'd join a union and start looking for a new job. That way if you refuse to work and they sack you, you have a union to help you take your (ex) employer to industrial relations tribunal and/or you would have an alternative job. Even if you on average don't exceed the 17-week average I'd consider changing jobs because an employer that forces you to work long hours is waving a red flag as to their suitability as employer already.
Be aware also that if they don't pay you extra you should check that your employer is then not in breach of minimum pay regulations... that's a whole new stick you can beat them with.