Hi Colinhann,
I don't know about the Dexcom7, but the other two meters you mention are not non-invasive. They both have a sensor that has to be inserted under the skin and this provides constant monitoring of interstitial fluid, rather than blood. This type of measurement is designed to give an approximation of blood sugar and is nowhere near as accurate as a traditional blood test monitor.
The Abbot website actually recommends that their monitor should not be relied on alone and should be used in conjunction with a normal finger-prick monitor. There were tests run on the Guardian monitor earlier this year by a Czech research team and their conclusion was
"The observed, clinically unacceptable broad limits of agreement do not support the use of the Guardian Real-time system for tight glycemic control management".
The "limits of agreement" that the research team referred to were a variance of between +2.3 and -3.1 mmol/l against true BS levels, hence their concerns about accuracy.
I'm afraid that I don't know if any of these is available on prescription, but in view of the reluctance of PCTs to allow patients to test at all, I would be very surprised if these are available on the NHS.