Many years ago I did a study of the Wiltshire Abbeys for a diploma in archaeology, it was'nt very good but it gave me a good insight into the life of the abbey.
The nearest ones to Avebury are of course Stanley Abbey, on the other side of Calne, excavated in the 19th century, Bradenstoke Abbey, near Lyneham, and maybe Lacock Abbey.
It is recorded however that there was a small 'alien' priory at Avebury, probably with only two monks, though the fact that there is only two 'proper' monks there might obscure the fact that their may have been lay monks and servants, and in their accounts they seemed to have owned 750 sheep, which would mean that they had plenty of land. The priory seems to have been where the manor house is now.
These monks came from Rouen, and were from the Benedictine Order, but the fascinating thing is, that their Mother house was also founded on a pagan site, presumably a Gallic settlement with a temple.
The abbey of Saint Georges de Boscherville is located in Saint Martin de Boscherville, near Rouen. Boscherville was a pagan place of worship at the end of the first century AD. Abandoned in the third century, the first temple was converted into a funeral chapel in the seventh century probably dedicated to Saint George
The cloister was excavated in 1981, and it was here that evidence of a pagan temple was found measuring 7.6. x 8.5.m, the most recent votive offering deposit dated from the 3rd century.
There had been a long going dispute between the Parish church and the Priory At Avebury about tithes and land, the parish church belonging to Cirencester Abbey, and eventually the priory seems to have disappeared, the following gives a list of the meagre possessions held;
Beside the farm stock and store of grains there was a horse for the prior and one for his socius. Indoors the brethren had a missal, a breviary, furnishings for their chapel, two beds, tables, and kitchen utensils. In 1324 they had also a chess set, the only luxury in their simple and lonely life.
From: 'Alien houses: Priory of Avebury', A History of the County of Wiltshire: Volume 3 (1956), pp. 392-393. URL: http://www.british-history.ac.uk/report.aspx?compid=36574. Date accessed: 06 April 2008.
There seems to be more pagan shrines in Gaul, than have been found in England., but mostly one could say that there is a strong pagan tradition in the West country, exemplified by the roman shrines found here and that when the breakdown of the Empire came about these shrines would still have been in use. That they are shown to exist is dependent, on what iron age hillforts have been excavated, and the fact that roman shrines were often intergrated on top of the old celtic shrines. The roman shrines often show destruction at a later date and evidence of 'new' shrines being erected as at Uley, the following link gives some of the shrines found in Somerset, Maiden Castle being similar in size to the Boscherville one, but Avebury has no evidence of a later iron age shrine.
Robert Vermmatt in his Vortigen Studies has written an essay on the possibility of a 5th century book being written in this part of the world, the book is housed at the Vatican, this an illuminated book with strong similarities in the artwork of the various roman mosaics in this area, and lettering similar to that found on a 'curse' stone at Bath. He puts forward the theory, that with the strong defence of the Wansdyke that there must have been a powerful tribal overlord in the area, and that the decapitated heads found at Uley and Bath may represent 5th century christian zeal in the destruction of the shrines.
There is strangely a strong sense of paganism in this part of the country, Ann Ross in Pagan Celtic Britain has many illustrations of the strange and wondrous votive offerings found, including a phallic head further west and a head found buried as a foundation offering in a house at Camerton.