The question is surely more one of where he is in the UK all time list. There are some stand outs like Redgrave but for all his achievements, he competed in an event that very few people do and which really only gets publicity for a race once every four years. His success has also been helped by able partners.
Tennis is played in a large number of countries by large numbers of people (or at least large numbers have played it at some stage and those with the talent will have had opportunity to emerge). It is an individual game requiring a challenging combination of fitness, skill, tactical awareness and determination. In addition, to get to number one in the world you have to demonstrate a consistently high level of performance. Murray may have just reached number one but he has been in the top four or five for some years playing in an era with 3 of the all time greats. Not only has he reached number 1, he has won slams, Olympic golds and the Davis cup. It really is a pretty impressive record.
Who else in British sport compares? Personally I would rule the cyclists out. Whilst I agree that Hoy has been outstanding, several cyclists have come to the fore and I think that a lot of their success is due to the advantage of the technology and the conditioning of the team for the big events. They are not performing at a top level week in and week out. Golfers also compete in an internationally popular sport week in and week out and so I guess Nick Faldo with his 6 majors must be up with the best.
Football must be the game with most players across the international stage but whilst we can think of loads of very good players (Best, Charles, Charlton and Law - to pick one from each of the home nations) I can't think of any players that would clearly fit the bill as a stand out player.
It is difficult to look at any name and say that they are ahead of Murray on the all time British list. If Murray can maintain his number one status for a couple of years and win another 2 or 3 majors then surely there will be little room left for debate.