Should you wish to go down the oil cooler route, I would 100% recommend getting the thermostatic.
As mentioned on our mishimoto thread, and as Rob mentioned, without the thermostat, your car may run colder than it should, with the UK weather and casual road driving. A thermostat basically helps regulate the temperatures keeping them in the optimal range, as opposed to the 'always on' option of a non thermostatic. (an application suited to track only cars that are always on the limit or hot climates)