This question comes from Steve in the USA - Hope this helps.
A. The first thing to point out is that you must distinguish between 'England' and 'Britain' at this point - despite the irritating recent tendency by historians to lump the British Isles together as though medieval England and Scotland, for example, are comparable. There is a scant amount of evidence for a Jewish presence in Wales but this isn't particularly significant or even conclusive and also evidence that some Jews were involved in the funding of the invasion of Ireland (though it doesn't naturally follow that they subsequently migrated there), so my answer is based on the English evidence. Figures are notoriously difficult to come by for the medieval period (and accurate ones even more so). However, it has been suggested that it's height in 1200 the Jewish community probably numbered no more that 5,000 and that figure was to decline by 1290, for various reasons, to 2,000-3,000. To put that into some kind of crude perspective, that's roughly 1.25% of the urban population or 0.25% of the overall population. Moreover, it should be noted that the population of medieval England was still, on the whole, agrarian in nature while a (growing) minority throughout the period would have been urban dwellers. As a result Jews composed a minority within this minority and, by extension, only a tiny proportion of the population would have come into daily contact with Jews.
UPDATE (25/4/16): Dr Pinchas Roth has kindly drawn to my attention that the use of the 0.25% figure is not entirely accurate. The reason for this is that I erroneously failed to specify that that figure applies to the size of the Jewish community in 1200 (this is based upon the figure provided in Vivian D. Lipman, 'The Anatomy of Medieval Anglo-Jewry', Transactions of the Jewish Historical Society of England, 21 (1962-1967), p. 65). By 1290 this figure was probably more like 0.1%.