I agree with Sqad that the cost of ambulatory blood-pressure monitors is prohibitive, although requesting one might be acceptable if your local hospital is a large one and/or you happen to inquire at a time when the devices are not being used by other patients. Yes, in an ideal world, the hospital should have written to your GP recommending hypertension therapy, but in reality, that letter may have not gone out yet or it may have been filed away or put on the practice database for the GP to see the next time he sees you.
White Collar syndrome is undoubtedly overplayed to my mind as well. I think the best thing you can do is to ring your GP Surgery and ask if they've received a report from the hospital regarding your hypertension and the postponement of your operation. When they say they have, make an appointment with your GP and discuss the matter with him or her. The GP will undoubtedly take your blood pressure again and if the hypertension is confirmed, they'll begin the appropriate therapy.
If the surgery hasn't received a report, get in touch with the hospital again and ask them to write to your GP about your hypertension and operation. Explain that you're in limbo until the GP receives the report. The consultant's secretary would be the ideal person to speak to about this.
A lot will depend on the urgency of the operation, but by working with your GP, the hypertension should come under control fairly quickly enabling your suitability for the operation to be re-assessed by the hospital.