In the Neolithic period, it is believed that emmer wheat was crudely ground up to make a thin gruel-like porridge. This is probably the way the fruit of arable farming was consumed throughout Europe back then.

From an Irish perspective, the Bronze Age bronze cauldron (which was exported across Europe, including obviously to Britain) was used for boiling food. Ornate flesh-hooks (used to take meat out of the boiling water) have also been found.

Also in Ireland - not sure if this extended to Britain, but Bronze Age contacts meant a wide diffusion of ideas between the two islands - the 'fulacht fiadh' was popular. This was basically a timber-enclosed hole in the ground. Water was poured in, and stones heated on a nearby fire were dropped in to slowly bring the water to a boil. It is believed that this was used to cook meat, though it may also have been used for other purposes.

Sources: JP Mallory, The Origins of the Irish Barry Raftery, Pagan Celtic Ireland Norman Davies, The Isles