I think you are misunderstanding the meaning of marriage *banns* (note spelling). This word does not mean "prohibition" - it refers to the official proclamation of a couple's intention to marry and was required in advance for all marriages (except where a licence was obtained) on three consecutive Sundays prior to the marriage.

The purpose of banns was (and still is) to advertise to the congregation that a couple intented to marry, in order to give an opportunity for someone to object if they new of a reason why it should not happen - existing spouse, lack of consent for an under-age party, prohibited relationship between them etc.

It is possible that your couple ran away to the big city to marry because there would have been an objection back home for one or other of the above reasons, or perhaps just because they families were dead set against it. If one or other of them was under age they could easily have told a fib and no one would have been any the wiser. Many large London churches had a reputation for asking few questions. But it is also possible that they were both working in London (perhaps as servants), married there and returned to Berkshire to set up home.