How can I cut a tree trunk into more manageable pieces?
I don't know about the UK, but in the US I'd look in the phonebook or online for "portable lumbermill service". There are indeed devices which can be brought to a jobsite and set up to slice boards from a felled trunk. Definitely worth considering for hardwoods, if you have a place to store the resulting lumber until it dries (typically a year per inch of thickness for air-drying). Cost per board-foot is usually quite reasonable.

The tools can also be purchased, of course. Some are as simple as a guided chainsaw with a longer than usual cut and a chain ground specifically for ripping. Others are guided bandsaws. The guides can be as simple as rollers following the surface of the previous cut (using a 1x12 nailed to the log to guide the first cut), or can be full frames with built-in tracks for the saw and mechanisms to assist with rotating the log to obtain the best grain patterns.

Or, if the wood's at all interesting, you could see if any local sawmill would be interested in buying the logs, and ask them if they could cut the slabs you want as part of that deal.