That is certainly the case for the vast majority of roundabouts. But there are also a number of roundabouts which have a large diameter, limited visibility and a continuous flow of fast-moving traffic and you’d be sitting there forever if you couldn’t nip into a small gap. I’ve noticed this in particular while in Kent in the south-east.

(I’ve driven across a decent amount of the UK, from Heathrow to Durham, Edinburgh, King’s Lynn, Greater London and Folkestone. I have an extended array of in-laws in Consett, Snettisham, Isle of Man, and elsewhere.)