For me, Jonny Wilkinson (despite Highlander's assertions that "team" sportsmen shouldn't be in there).
There is nothing about that man that does not command respect as the best we (Britain) have ever had.
New Zealand's Dan Carter pushes him close in terms of ability - maybe even better but as a New Zealander he's not qualified.
Jonny Wilkinson is dedicated, wholly professional, successful and a role model for all.
Andy Murray may well be (Hoy et al aside) Scotland's "greatest" sportsman.
Never Britain's greatest - "ABE" - (anyone but England) - being a great sportsman goes beyond being successful in your chosen sport.
It's Britain's greatest sportsman you're looking for - not the most successful in his chosen field.
If that was the case you need look no further than Phil Taylor.