The problem with your question is many-fold... it is a reasonable question but can have a load of differing answers...... Dave, let me explain it like this.......

If RTW Rider 'A' said "No way it cost me £26,458" you would be disheartened and maybe feel like giving up......

alternativeley, if RTW Rider 'B' said " I did the lot for £11.250 " then you would feel like you had way more than you needed.

The thing you have to look at is this, read as much as you can on this site, on other sites too, ( even Grant the owner of this site looks on other sites for information.....) compile a general idea of what it will cost you for your daily living in each Country, UK & europe expensive, africa cheaper...etc etc
Figure out your costs for Bike transportation & shipping ( don't think about paying bribes to border guards, it shouldn't have to happen, if you do, you make it harder for those that follow, but if you have to, the less the better.ok? look on here at the many threads and postings on that subject!)

Work out a route that suits YOU. its your Journey, your trip, your experiences & your money...... don't let others spend it for you!, take advice, listen to everything and select what suits, even what I am telling you, maybe you will or wont, thats your prerogative.

£15,000 is a good chunk of cash.. the further from the UK or Europe you get, the more in real terms its worth so the further it should take you.

Always always always have an Emergency/ Contingency plan though.....

Martyn