Couriering will certainly change your approach to bikes, I got into it for similar reasons to you, loved bikes and fancied riding for a living.
At the moment work is very sparse and a lot of firms aren't taking on riders so you can't expect to earn very much.
It takes at least a few months (as a London courier) to get a reasonable working knowledge of the different areas around the capital, until you've gained experience it'll be difficult to earn a reasonable wage. When I began I had a second job to prop up my income.
At best you may be able to earn about 600 for a (very) busy weeks work, although between 3-450 a week is more typical. A lot of this depends on the firm you work for though, small firms generally don't have as much work so income will typically be lower.
It's very expensive whatever bike you ride, when you consider you'll be riding 30000 miles a year that's a lot of petrol, tyres and parts needed just to keep on the road. Hours are long, usually ten hours a day minimum...
It can be great some times though things like bad weather, driving and stress can put a damper on things - and you'll have to endure a lot of cold, damp weather! Customers can be less than appreciative of your efforts too.
The majority working in London these days are Brazilians which has an effect on levels of pay and working conditions.
If you want to have a go I'd say do it, but don't expect to earn very much at the beginning and it perhaps isn't best seen as a long term career these days - though saying that I've been at it the last 4 years.