This isn’t a UK case, but I could see the same happening here — as the article states, all the elements of a contract seem to be present:
Many people don’t quite realize that almost any kind of “agreement” can be seen as an enforceable contract in the eyes of the courts. While some people think a contract has to involve a full written document and signatures, that’s often not true at all. Take, for example, a case involving affiliate sales of e‑cigarettes, in which a written contract was deemed to be modified by a simple instant messenger conversation. The affiliate company, CX Digital, wanted to remove the contractual limit of 200 referrals per day for sales of Smoking Everywhere’s e‑cigs, and the following IM conversation ensued:
[CX] (2:50:08 PM): We can do 2000 orders/day by Friday if I have your blessing
[CX] (2:52:13 PM): those 2000 leads are going to be generated by our best affiliate and he’s legit
[Smoking Everywhere]: is available (3:42:42): I am away from my computer right now
[CX] (4:07:57 PM): And I want the AOR when we make your offer #1 on the network
[Smoking Everywhere] (4:43:09 PM): NO LIMIT
[CX] (4:43:21 PM): awesome!
Smoking Everywhere then tried to bail out on paying CX the affiliate fees owed, and claimed (among other things) that this didn’t represent an actual modification of the contract which had the 200 per day limit stated. The court didn’t buy it, noting that this is a perfectly fine example of an offer, counter-offer and acceptance, all encapsulated in instant messenger.