The House of Commons has the power to suspend MPs who violate parliamentary standards. MPs who are suspended for 10 days or more can be subject to a recall election under the Recall of MPs Act 2015, ejecting them from parliament.
Rules 10 and 11 of the House of Commons Code of Conduct say:
No Member shall act as a paid advocate in any proceeding of the House.
The acceptance by a Member of a bribe to influence his or her conduct as a Member, including any fee, compensation or reward in connection with the promotion of, or opposition to, any Bill, Motion, or other matter submitted, or intended to be submitted to the House, or to any Committee of the House, is contrary to the law of Parliament.
Section IV.7 says:
In carrying out their parliamentary and public duties, Members will be expected to observe the following general principles of conduct identified by the Committee on Standards in Public Life in its First Report as applying to holders of public office. These principles will be taken into consideration when any complaint is received of breaches of the provisions in other sections of the Code.
"Selflessness
Holders of public office should take decisions solely in terms of the public interest. They should not do so in order to gain financial or other material benefits for themselves, their family, or their friends.
Integrity
Holders of public office should not place themselves under any financial or other obligation to outside individuals or organisations that might influence them in the performance of their official duties.
Objectivity
In carrying out public business, including making public appointments, awarding contracts, or recommending individuals for rewards and benefits, holders of public office should make choices on merit.
Accountability
Holders of public office are accountable for their decisions and actions to the public and must submit themselves to whatever scrutiny is appropriate to their office.
Openness
Holders of public office should be as open as possible about all the decisions and actions that they take. They should give reasons for their decisions and restrict information only when the wider public interest clearly demands.
Honesty
Holders of public office have a duty to declare any private interests relating to their public duties and to take steps to resolve any conflicts arising in a way that protects the public interest.
Leadership
Holders of public office should promote and support these principles by leadership and example."
Taking bribes to switch party would seem to violate the rule on bribes and paid advocacy, and go against principles of selflessness and integrity. The procedure is that MPs are referred to the Commons Select Committee on Standards which collects evidence and makes a recommendation. Parliament may then vote to suspend the MP. If the MP is suspended for long enough, constituents can then petition for a recall election, after which the MP may be replaced in Parliament.
There are allegations that decisions have been made on political grounds when it comes to votes in Parliament about whether to suspend, as with the controversy over Owen Paterson in 2021. But many Conservative MPs would doubtless want to punish anyone who left the party, and opposition MPs would vote for a suspension.
It is a basic principle of parliamentary democracy that the courts have minimal say in what MPs do, so MPs are very unlikely to be prosecuted for bribery, but parliament has the ability to sanction MPs who break parliamentary rules.