**Summary**

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 it 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.

**Two Possible interpretations of the Poster’s Question**
 
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](https://www.easyjet.com/en/policy/coronavirus-faqs).

**1. ‘Rain check’ as an exclusively US English expression**

This is the position supported by dictionaries I have consulted. For example, the *Chambers Dictionary* (iPhone edition) has:

****raincheck** noun (US)…**

and the [Cambridge dictionary on line](https://dictionary.cambridge.org/dictionary/english/rain-check) has:

**rain check** *noun* US

(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 writing*, a British person ‘should’ spell the American expression exactly as it is spelled in the US — **“rain check”** (or raincheck). 

What is the evidence for what I say is customary practice? Consider the word “labour” (British spelling) or “labor” (US and Australian spelling). Whereas US newspapers (e.g. the *New York Times*) refer to the “British **Labour** Party” as the [“British **Labor** Party”](https://www.nytimes.com/1927/10/04/archives/british-labor-party-expels-communists-blackpool-conference-approves.html), British newspapers (e.g. *The Times*) refer to the [“Australian **Labor** Party”](https://www.thetimes.co.uk/article/support-for-australia-labor-party-at-lowest-in-40-years-gs77zl0nllb) with the modern Australian spelling.

Likewise the spelling of “centre” (US “center”). *The Financial Times* (London) refers to the [“World Trade **Center**”](https://www.ft.com/stream/d3ab542e-8aab-4afb-b7dc-e841302b96b0), although  if they referred to its location in Manhattan in the same article they would use the spelling **centre**. 
	

**2. ‘Rain check’ as a British English expression**

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, this argument is based on the 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:

> **check** (noun) 14 
> 
> b. A token, usually a memorandum of receipt, a ticket,
> a piece of metal duly stamped or numbered, used for the purpose of
> identification, or as evidence of ownership or title: given, e.g. to
> the owner of luggage on the railway…

with an early example quoted:

> **1847** *Illustr. Lond. News* 4 Sept. 146/1 They will deny the receipt of a
> **check**, and exact the fare again.

As a footnote one might reiterate the catholic attitude of the British to the spelling of imported words by mentioning the adoption of the US spelling “program” in the context of computing, even though one would write about the “programme” of a meeting on the topic.

**One final check**

I find the idea of a spelling “rain cheque” quite bizarre. The modern use of this spelling in Britain is, in my experience, exclusively for *financial* cheques. It is ironic that someone should want to adopt a spelling found only in something that is rapidly disappearing. Is it for a wish to preserve a word from extinction, or is the poster is too young to know what a cheque actually is?