Regarding accuracy of "boots devices", CAA says home blood pressure monitoring is acceptable. Their procedure is: "for each blood pressure recording, take 2 measurements 1 minute apart, take 2 recordings a day for at least 4 days, discard 1st day measurements and use average value of remaining measurements" (details:
http://www.caa.co.uk/docs/2499/Hypertension%20FC.pdf )
Re Salt (Sodium), it makes your body hold on to water. The excess liquid in the system requires your heart to work harder, raising pressure. Watch out for processed/"industrial" food, a lot of salt is hidden there. For cooking at home, there are low-sodium alternatives, which taste the same, such as:
Blood Pressure : Low-sodium salt alternatives
Re caffein, check your pressure within 30 minutes of drinking coffee or another caffeinated beverage you regularly drink. If your blood pressure increases by five to 10 points, you may be sensitive to the blood pressure raising effects of caffeine.
Re age, blood vessels can become thicker and less flexible. The stiffer vessels no longer expand and contract as much with each heartbeat, thereby increasing opposition to the flow of blood, placing an increased workload on the heart, increasing systolic pressure. As with any pump, accelerated fatigue is an issue if the heart is constantly working harder than what it was designed for, and the less flexible vessels, under higher pressure than they were designed for, are at greater risk to rupture. Tracking the issue and getting it under control at an early stage, preventively, before any damage is done, what you are doing now, is a good idea. The threshold for what is defined as high blood pressure, for what puts you at risk, does not change with age, it's still high blood pressure. Some experts recommend a slightly relaxed threshold before putting older individuals on medications, yet that's completely irrelevant for a CAA medical. All that matters for a medical is the CAA guidelines. Some claim specific stretching exercises may help to keep arteries flexible (and again, diet can play a role too).
Excess Salt, Alcohol, Sugar, Caffeine etc...there's a lot of good advice here (mostly in line with advice from
http://www.bloodpressureuk.org/Home ), and I think with the help of your home blood pressure monitor, and a log, you will figure out what works for you, (we're all different and have different habits, different diets, and may react differently to substances and/or treatments).
"This morning's 'Boots' figure was 138/79". Congratulations, that's not dramatic at all. So before you measure your blood pressure next time, take a few deep breaths, exhale slowly, close your eyes, think of something relaxing...that may also lower blood pressure. Then, enjoy flying.