Hi there; this link may be of some interest, for the UK end. At normal jet altitudes, the radar coverage will be greater than shown, as it's basically operating line-of-sight - as does VHF radio.
http://www.nats-uk.ead-it.com/aip/cu...6_1_6_2_en.pdf
If you look at
ENR 6-4-0-2 in the same AIP document, there's a chart of radio coverage over Canada; I'd guess that the chart for radar would be somewhat similar, but broader at height.