The poster does not make clear whether he wishes to write the phrase as a consciously reference to a non-British English term, or as a foreign expression that is entering or has entered British English. I argue that in either case second portionit should be spelled “check” primarily because British English customarily adopts foreign spellings, but also because the spelling “check” as a general term for a counterfoil has long been in use in Britain.
I’m British too, but I am unsure in what context the poster wishes to use the expression — as a recognized non-British expression, or on the supposition that the expression has entered or is entering the British English canon. I shall consider both possibilities, although the wording of recent offers of alternatives to refunds for cancelled airline flights shows no evidence of the latter.
(Others have given the definition, so I will assume it is known.)
In this case I argue that if one follows what is customary in British writingif one follows what is customary in British writing, a British person should‘should’ spell the American expression exactly as it is spelled in the US — “rain check” (or raincheck).
What is the evidence for thiswhat I say is customary practice.? Consider the British English spelling ofword “labour” (British spelling) or “labor” (US and Australian — “labor”spelling). Whereas US newspapers (e.g. the New York Times) refer to the “British Labour Party” as the “British Labor Party”, British newspapers (e.g. The Times) refer to the “Australian Labor Party” with the modern Australian spelling.
Likewise the spelling of “centre” (US “center”),. The Financial Times (London) refers to the “World Trade Center”, although if they referred to its location in Manhattan in the same article they would use the spelling centre.
If one considers, however, that the US expression has entered or is entering British English, then the argument for the spelling “rain cheque” depends on the idea that the spelling “check” is not British in the sense of a voucher or receipt, and that in order to be assimilated into the language the spelling should be changed to “cheque”. However
However, this argument is based on athe false assumption that such a use of “check” is not British English. Although in this context the spelling “cheque” is older than that of “check”, OED has the following:
I find the idea of a spelling “rain cheque” seemsquite bizarre. The modern use of this spelling in Britain is, in my experience is, exclusively for financial cheques. It is ironic that someone should want to useadopt a spelling found only in something that is rapidly disappearing. Is it for a wish to preserve a historic word from extinction, or is the poster is too young to know what a cheque actually is.?