When you say the beds will be empty do you mean of plants or of soil? I live in Mangotsfield and had that red clay soil but dug it all out of my raised beds when I removed railway sleepers & used the soil for raising the lawn area. (Organo-Phosphates [the preservatives used in rly. sleepers] can be real nasties and shouldn't be used for veg plants) so I changed the sides & supports for tanallised boards & steel channels. My 3 main raised beds are 4 ft.x10 ft x 14inches deep and they took 4 'builders bags' of topsoil to fill them all. I bought nice friable loam topsoil from a company in Solihull who were advertising on the internet. This was excellent except for a few nettles that began to come up in it but judicious weeding has solved that little problem (just glad it wasn't mares tail that someone else on this forum got in 'fresh' topsoil!)
If you just want to add manure to existing soil then the Bristol soil (assuming yours is similar to what mine was) then stable manure will help break up the clay and will give you good results (though be warned - horses eat weeds as well as grass and the seeds come out at the tail end already primed for a robust life wherever it drops! A Cambidgeshire farmer friend once told me to lighten clay you need horse manure with plenty of straw in the mix but with light sandy soil you need cow manure to make it more workable and retain moisture.
My guess is that 1/2 a dozen 'builders bags' of well rotted stable manure will enrich the ground well but you may not need to manure all of the beds in one year. Root crops will not benefit from fresh manure as it will cause them to fork and distort. Beetroot will just produce leaf growth & little root if the soil is too rich. Legumes (Peas & beans) will love well manured ground as will brassicas (particularly cauliflower and broccoli) + cougettes, marrows, salad leaves etc. Potatoes will like a little organic material as this stops 'scab' on the tubers. Runner beans will benefit from manure and the addition of anything that will hold water [straw etc] as they thrive in moist well watered and manured ground.
My method for manuring a bed is to dig a trench at the end of the bed & put the soil at the back, then fill the trench with manure and backfill with the next trench on top to form a new trench and work backwards through the whole bed with the original trench material going on top of the final addition of manure. (My dad taught me that in the 1950s). Hope this helps, John H