To get a thunderstorm you just need a thick depth of cloud (usually to the troposphere) with cloud tops less than about -20 celsius and convective enough to sustain hail growth. (this is not quite the whole story…but sufficent for this)
Southern england tends to experience thunderstorms with the release of medium level instability from northwards tracking tropical airmasses, so people in southern UK tend to associate thunderstorms with hot summer days,
Across Northern Britain, this tropical outbreak triggering situation is much rarer, but very cold air moving across warm sea temperatures can trigger very deep convection as sockpuppet describes, so in northern half of the UK thunder tends to be a winter phenomena…especially around Northern Scotland/North Isles.
Haldon Hill last year was a nice example of severe convection in cold air being triggered by warm sea temperatures around Cornwall.