This sounds a bit embarrassing to ask actually.

Is there any source of 'travel grant' in the UK, for which you can apply *after* the travelling arrangements have been made?

I'm a postdoc out of work currently and recently an opportunity has come up to spend a couple of weeks as a visitor at a foreign institution. I really want to go because if things work out reasonably well, they might even offer me a job later. So I'll need the money to cover the tickets, plus the accommodation costs for two weeks, plus some bench fees for that institution.

The problem is how to find all that money. All the sources in the UK that I know of require that we apply 2-3 months before the visit, which I can't do now because all this has worked out only recently.

Does anyone know of any sources of travel grants that are flexible enough for this sort of situations?

There are funders listed online but they are ear-marked for specific groups (e.g. women, medics, residents of Bradford etc) so I can't recommend any as I don't know your specific circumstances. Ask your librarian for assistance finding information about trusts and charities. There are agencies that offer interest-free loans (i.e. they give you a 'grant' that you have to pay back when you can afford it) if that is feasible for you. I'm self-funding a full-time PhD and looking for finance to attend international conferences - tall order but not impossible if you think laterally. Good luck!