As far as the U.K. is concerned, all cellphone numbers are now:
+44 78... or
Other +44 7x assignments are used for personal numbers, pagers, etc.
You can get a complete list of the U.K. cellphone operators and the codes assigned at the Ofcom website
. (Look for the Excel-format spreadsheet link a little way down the page.)
You can find links to various numbering guides for other countries at the World Telephone Numbering Guide
With regard to North America, it's not so simple, because cellphones are not assigned their own "area" codes as in Europe, e.g. the 212 area code serves both landlines and cellphones in Manhattan (and American area codes evolved on rather different lines than in Europe anyway, but that's another story!).
American/Canadian cellphones are assigned exchange prefixes within
their own local area code, so you need to look at all six digits (area code plus 3-digit exchange prefix) to identify a cellphone number.
One of the main reasons for wanting identify cellphone numbers this side of the Atlantic is the high cost of calling them, and that problem doesn't exist in North America, where it costs the caller no more to call a cellphone than to call a landline in the same location. My apologies if I'm stating the obvious, but a lot of people over here don't realize there is a difference in the charging arrangements.
If you really need to find North American assignments, go to the "Numbering resources" section of the North American Numbering Plan
administration. From there you can search any given NPA for all the exchange prefixes assigned.