Tyres.co.uk sells the Michelin Agilis 3 and Goodyear EfficientGrip Cargo 2 in 215/65 R16C 109/107T for 310-series vans as well as Ford factory fitments in 215/65 R16C 109T. Make sure to check the load and speed ratings on your registration as some series of the Transit may have different commercial-tyre specifications.
The tyre size for the Ford Transit is located on the tyre pressure placard which is found at the driver's door opening, on the B-pillar or on the edge of the door. It lists the approved tyre size, the load index, the tyre speed rating and pressure corresponding to axle load. Refer to your handbook and check the placard as the placard may not be primary evidence.
For a Ford Transit, it is not usually necessary to fit all four tyres. Tyres should be replaced on an axle to axle basis provided that all are of a matching size, construction, load rating, speed rating and so on. For dual axle fitments in the UK, it is recommended practice to fit the newer tyre on the rear axle. For AWD Ford Transits, all four tyres should be replaced when there is a significant difference in tread.