I thought that one of the MHRA documents released yesterday stated that a prescription would be required to purchase any e-cigarette products that are licensed by MHRA.
Here in the US, drugs are typically approved by FDA to require a prescription. If the company submits additional safety evidence about the product to FDA, the FDA can approve it for OTC sales. That's what happened with NRT, which required a prescription back in early 1990's, but was approved by FDA for OTC sales around 1995 (as I recall sending comments to FDA urging them to approve OTC sales of NRT to increase their usage by smokers).