What legal ... recourse is possible?
One could make a complaint to the Advertising Standards Authority (ASA) being the UK’s independent regulator of advertising across all media.
The ASA enforces the following Codes depending on where and how an advert is published:
Both Codes, sections 3.33 to 3.37, cover instances of "Comparisons with Identifiable Competitors" and use identical wording to require either "Marketing communications" or "Advertisements" (respectively):
... that include a comparison with an identifiable competitor must not mislead, or be likely to mislead, the consumer about either the advertised product or the competing product.
The ASA has a variety of sanctions available to it to deal with Code breaches ranging from damaging an offending advertiser’s reputation via bad publicity to referring the case to either Trading Standards or Ofcom who have their own statutory powers - including the possibility to prosecute serious or serial offenders.