Most of the larger insurers (e.g. L&G) won't load the premium if you fly under 100hrs/year as a private pilot (no commercial work).
There are some specialised life insurance brokers advertising in the back of the UK aviation mags who claim to have negotiated special deals but in reality they just take advantage of the above.
It is said that if AFTER having taken up the cover you then start flying say 500hrs/year the cover remains valid but I would guess, in line with normal insurance full-disclosure rules, if there is a claim and they choose to look at your logbook and find you were flying 200hrs/year at the time of the application, they will probably try to walk away from it completely (as they can under UK insurance law). The same comment would apply to e.g. risky sports taken up after starting the insurance; you remain covered, if something is not specifically excluded then it is included.