Considering that this is very close to the National Cycle Route 51, I'm wondering why it's basically impossible to get to, on a bicycle without going on a major trunk road roundabout?

Cyclists may not be their target market, however service stations can be useful on longer cycle rides for food supplies. Also hotels can prove useful if a longer journey, touring, or realise your too tired on an overnight ride, such as the nearby Dunwich Dynamo.