Should they be willing to prescribe Propecia, yes I would be totally honest and explain how you are planning on taking it and discuss any concerns you may have. (Though I suspect they will probably recommend you take the full 1mg for the first month to see if you encounter any problems....)
From the consultants I have spoke to, they do seem to be genuinely helpful/friendly and do offer valuable input. As i stated earlier though, it is imperative that you get all the clear before you jump on the Propecia (and Regaine!!) bandwagon. The LAST thing you want to do is take anything that may jeopardise your health.
Regarding the regaine, although I seem to remember reading something about it effecting blood pressure, i'm still not 100% sure. Again, this is something you should definitely discuss when you have your initial consultation. If the Boots consultant is unable to advise you, speak to your local GP.