GPs are loaded.

GPs, it depends.  If they're partners (I don't think John would be) then it could be quite a lot.  But it would be well above the national average.   A part-time practice nurse would not get paid much.  John might have some money if he was medically discharged from the army - not sure about that.   Between John and Mary, they'd have a decent income, but yes, living in London is very expensive and the wedding would have cost a bomb. 

Alll of them (Sherlock, John, Mary ... Mrs Hudson) seem to wear more expensive than average clothes.

I had the same idea, Mary. 

I was just going to suggest that Mary has plenty of money from her past hit-jobs and that she lied to John that she inherited all the bucks from her wealthy American aunt. 

I think it would be unusual (or unexpected) for those characters.  I don't get the idea that they're craving public attention, they have no family to invite (apart from one person who doesn't attend), few friends of their own.

SusiGo wrote:

I am not a native speaker myself but I learned on this board that Sherlock's "laterz" and "blud" are regarded as highly colloquial and not typical for his manner of speaking. Is that right?
 

 
Definitely colloquial and "OOC" for him, which is probably why he said it in the first place - for effect/shock, and to purposefully annoy Mycroft.

As for the other things...

Money
Mary probably has money of her own in her bank account saved up - possibly from her assassin jobs!

John will have an army veterans pension that goes steadily into his account and although not a vast amount, that will top up his earning as a GP. I know he couldn't afford to live on his own when he first moved to London but that was possibly more being unable to actually SUSTAIN living on his own without a job for any long period of time. He still would have had some disposable cash in the bank. Soldiers don't actually spend very much at all when they're abroad so it sort of just sits in their bank accounts and builds up.

John also would have earned cash from his jobs with Sherlock. I very much doubt that Sherlock would have just kept all the money to himself. Sherlock isn't interested in money particularly and would have been happy for John to split it seeing as they were essentially partners working together. And if you think of the amount of money they earned in Blind Banker you can imagine that some of their private jobs were very lucrative.

By the time we see John in TEH he has settled down and obviously has a full time job working as a GP. We don't know whether he runs his own practice or whether he just works at a general NHS practice but either way GPs earn a very decent amount of money, particularly ones living in London. YES, it's much more expensive to live in London, but all of the jobs are better paid in London too, so it matches up and works out OK.

They wouldn't have had any problem paying for the wedding. That would have just come out their earnings without them really noticing it much. Buying a house and getting on the property ladder is difficult, but with John being a steady earner in a respectible profession, they would definitely be able to get a mortgage, which is probably what they have. The car is a nice one but again, how much is it, really? £30,000 maybe? That's less than a third of John's yearly wages. They can afford it.

Class/Background
I always assumed Sherlock was from a posher background than John too, that was until I saw his parents! I actually think it's a rather neat idea that they've gone for. Mummy and Daddy Holmes are basically an ordinary couple who have some extraordinary genius kids. They're clearly not struggling for money as they own a nice little cottage in the countryside, but they're not sprawling estate type people either. They're not upper middle class. They're middle class. It's quite possible that both Mummy and Daddy Holmes came from working class families and they bettered themselves and worked up to have good careers and earn decent money. They actually remind me a lot of my own parents, who did exactly that, and are now middle class (which makes me middle class too).

Sherlock is quite clearly frivolous with money and he doesn't really care about it, whereas John seizes on it when it comes along and makes sure they actually earn. That makes me think John probably comes from a family where money hasn't always come that easily ie. a slightly poorer family than Sherlock's. This is just my head canon but I've always imagined that he was the first one in their family to actually go to University.

Swearing
None of the swearing used in BBC Sherlock is particularly shocking to me. I should imagine that in "real life", they both use the 'f' word a lot more frequently, Sherlock less so than John.

I really dislike it when people use the word 'bathroom' to refer to the toilet in Britain. It's an American expression, I'm quite sure. I wouldn't call it a bathroom unless there was a bath in it. I'll always remember how amusing I found it though, when we went to America and my Dad went up to someone and said "where's the loo?" and they just looked at him like he'd gone mad and said "The what?" so my Dad said, "You know, the loo...the bog..." Still nothing. Finally, my mum had to go over and rescue him to ask for the bathroom.