They really can't. UK bookies are pretty tightly and strictly regulated by the Gaming Commission, which took over from the former Gaming Board about a decade ago.
The licensing procedures are also a huge disincentive for them to try to circumvent regulatory processes and procedures, because they have to apply to Magistrates' Courts once a year to renew their gaming licence, and on those occasions any members of the public who have a "beef" about them get the opportunity to air it in detail, in court, in front of all the media. There have been a few instances of bookies getting into serious trouble this way, and it's something they don't dare take a chance on.
They're also fairly well self-regulated, with an independent arbitration council whose findings in individual cases are binding on them.
Many professional and semi-professional punters say they'd certainly far rather have an argument with a bookie than with a spreadbetting company, and I can see why, from some of the cases I read.