I wouldn't worry about it. Firstly, it's fairly common. Secondly, if you really came under pressure to explain yourself you could say it was an effort to impress when asked about a new opportunity, or that you had something else in the pipeline on that wage, or you were due a raise, or that you misheard and thought you were asked what money you wanted.
If they've offered you an amount, they can afford it and they want you. It's not like you've lied about qualifications or having a criminal record or something critical to the job.
If you do really enjoy your current role, might be worth mentioning to your boss that an opportunity has come along at £XXXXX