British equivalent of "taking a rain check"

Is there a British equivalent of "taking a rain check"?

It would be very useful to have a phrase that I can say to British people which means "I need to cancel and I'd like to reschedule but I can't give a specific time right now."

Is there one?

Edit: This phrase is widely recognised in the UK but very rarely used. It is usually misunderstood as meaning "I'd like to reschedule (to a specific time)".

Solution 1:

"Can I think about that?..." or "Can I get back to you [on that]?" are probably the closest GB to the American 'Raincheck' - which I think is widely understood here.

"Can I think about / get back to you on that...?" is a polite way of avoiding an embarrassing situation, hopefully without hurting anyone's feelings, when: "You must be joking!" is perhaps a little too direct.