Hey Greg
If 'n when the moon pays a visit we will solve the world food shortage - let 'em eat green cheese.
I doubt it will have an effect on the moon unless you intend to be around a few hundred million years and a stray asteroid couild do that far more catastrphically.
The worst effects are the inevitable silting of the estuary above the barrage, there would be no Severn Bore, and wildlife would change as the Severn reduced in salinity &/or increased in polution. The inevitable motorway on the causeway would effect the landside wildlife by disturbing the birds etc and also on Flat Holme and Steep Holme (verboten to all but a few) - because that's the most likely route for a barrage.