Estefex was actually quite correct you must remember to qualify it within a probability of 'x' amount of miles of a given point, i would have said 50 miles would be correct, you can not predict much closer even the NWS and NSP centre in the US do not normally predict severe weather in less than that distance. the Oxford Supercell was almost within that distance of you. As to what you get with that type of storm even a mile can make a difference as to what you get one side of Bicester had 1" hail the east side just rain.
So on our small island draw a 50 mile circle on a map around Oxford that will cover a large area the storm top when near Oxford was so high it was visible from Portsmouth i personally watched it from near Brill in Buckinghamshire and at one time drove under the edge and got caught in a torrential downburst soon got out of it to watch it start to break down to the north east. Remember just because storms of any sort are predicted does not mean you will personally get your own