You will have to pay VAT at 17.5% as well as import duty. This seems to vary with value so your best bet is to give C&E a ring and ask.
If I remember correctly, last time I paid it, it was about 3.5% on a value of about £250 so if the saving is great then it can still be worth it.
WARNING
If you try and come back through the green channel and not declare it and then get stopped, they are within their rights to take it off you and keep it. At the same time, if they are having a bad day, they could even detain you which could involve a full strip search and internal examination as by this point you have been caught trying to evade tax and they don't like that.
This alone could be worth paying TAX & VAT to avoid