Well firstly OFCOM won’t be too interested other than to add your complaint to the ever growing pile they have labelled ‘Complaints about VM which one day we’ll get around to sorting out with them’
If you want to make a formal complaint about this then the industry regulator CISAS is the way to go.
In terms of a consideration, the other party, VM, have offered you a faster broadband speed, and in return you have offered to carry on paying them for a further 18 or 24 months minimum. So on that basis there would definitely be a consideration on both parties and it would certainly be enforceable.
Now did you understand this and was it explained to you that agreeing to the offer would start a new minimum term contract? Was it mentioned in the small print of this email? If it was and it was clear, then arguing that ‘well I didn’t read that bit’ won’t wash legally!