I had exactly the same problem - my BP was high on the test and they sent a letter to my GP to request 24 hour monitoring. Once that came back clear; I passed. That wasn't with TfL but I wouldn't imagine the procedure is too different. Also might be worth mentioning to the doctor/nurse in charge; from what I've heard it's quite common for those with no history of "white coat syndrome" to have elevated blood pressure on the medical as there's a lot riding on it.
Do you rememner how high?
Can't remember exactly I'm afraid but it was somewhere around the 150/110mark. Bear in mind that I was only 20 when I did it; so the limits could have been a bit lower.
Really try not to worry about it - worrying will only increase your blood pressure! There will be separate limits for the systolic and diastolic numbers - I'm not sure what they are exactly. Since it's now well established that hypertension cannot be diagnosed on the basis of one clinic visit alone, if it's high on the day they'll give you a 24 hour monitor - and if your BP is normal that will clear you. Relax - you'll be fine.
Besides, even if it is high, I understand that you can be on antihypertensives as a driver so presumably treating it is an option as well.