There is no legal height to lorries allowed on UK roads, there has been a trend to make taller lorries especially for lightweight goods.
They cannot make that bridge any higher (because the railway would require ramping for a considerable distance) and to make the road lower would require the bridge substructure to be rebuilt as well as numerous other engineering problems which would involve closing the railway and the road for considerable time.
The bridge is 4.7m which is lower than the suggested maximum height of a normal lorry (4.9m) and I think it is still only marked in imperial measurements (15'3") which may not be good for the numerous foreign drivers we have around. I also don't think it has signage warning of a low bridge.
There are numerous devices available that could warn drivers that their lorry is too tall.
Most European countries have a 4.9m limit.