While Chuck usually has good, reliable advice, perhaps a little additional information would round out his good intentions.
Teak wood has natural oils akin to cedar that gives it a natural resistance to weathering. However, there's no product that is made from actual "teak oil", at least not here in the U.S. Such roducts called teak oil are actually linseed oil or tung oil. Since it has no ingredients that protect the bench from ultra-violet (UV) rays the "teak oil has to be reapplied... probably monthly or so.
However, there are "Teak Sealers that do incorporate the extra protection. One brand I like is SEMCO Honeytone Teak Sealer. Maybe it or a related product is available in the U.K.
So.... look for a product called Teak Sealer (again, here in the U.S.) that has UV protection incorporated. I've also used a product called "Spar Varnish"... not an actual varnish but a marine grade protectorant that lasts a lot longer... One brand I use on boat teak is McCloskey Man O' War Marine Spar Varnish. It, too, will have to be sanded a little and reapplied, but only once every two or three years, depending on how much direct sunlight exposure it's subject to.
Best of luck!