The United Kingdom has no law that specifically outlaws Holocaust denial, with the Institute for Jewish Policy Research (JPR) stating, “Current laws on incitement to racial hatred in the United Kingdom do not have the effect of prohibiting the activities of Holocaust deniers.”[8] This is not to say that there have been no attempts at introducing legislation. In 1997 a Holocaust Denial Bill was tabled in the House of Commons which would have amended exciting legislation to criminalise Holocaust denial. The Bill failed however, due to insufficient parliamentary time. Interestingly, the Institute’s panel reached a unanimous view that criminalisation of Holocaust denial would be inadvisable.  Among their reasons were the potentially illegitimate infringement on the right to freedom of expression, unconvincing evidence from other jurisdictions with such legislation, conceptual problems, and a fear that prosecution of such cases could work to the advantage of deniers.[9] Instead, they recommended amending existing laws on racial hatred, “in order to improve their effectiveness in dealing with expressions of racism and anti-Semitism.”[10]

https://www.lawteacher.net/free-law-essays/human-rights/holocaust-denial-in-uk-8763.php

Either way, there are dozen of cases where they arrested people based on a technicality. In case of Alison Chabloz they just said Holocaust denial is "grossly offensive" and therefore illegal. Frederick Toben is another such case.