I've hired dozens of developers (in the UK and elsewhere), and I've
never taken an offer off the table because a candidate has asked for more. The worst I've ever done is not offer more than the initial offer.
There's almost always wiggle room - personally I've never accepted the first offer, and typically I've gotten 30%-50% more. Conversely, because I know there are people like me out there, I never offer the maximum I'm ready to pay - nobody has ever just quietly walked away if the offer wasn't good enough.
The only exception would be if I was hiring for specific very high level talent when the market is totally overheated.
> That's why I like recruitment agents, when you apply for a job, you already know the salary (or at least the salary range)
Recruiters are not your friends. While it is in their interest to get you the highest salary possible because it increases their commission, it is also in their interest to hire as many people as possible in as little time as possible, which means getting you to accept a lower offer if that's the quickest way there.
I am not saying not to use recruiters - I have successfully gotten got jobs via recruiters - but be very clear with them about salary expectations, and remember that just because a recruiter is involved does not mean the offer isn't negotiable.
On the contrary, if, when you get an offer, you make it clear you need more to accept right away, they will bat for you because they want that commission. Conversely, realize that they will at the same time bat for the employer if they think they can get you to sign right away. What matters to them is which side is most likely to cave.
Be nice to a recruiter and they also often will give you an indication of how far the potential employer is likely to stretch. I've several times had recruiters tell me "the instruction we got says X to Y, but I think they'll stretch to Z" where Z was a number I thought would've been outrageous to even ask for. In one of those cases, the company did in fact come back with an offer of Z after some strong hints + a very significant (far above normal levels) guaranteed raise at a specific later time.
Do practice interviews. Apply for a job you're not that interested in, and be actively unreasonable about what you ask for. You might sometimes find they don't think it's so unreasonable.