All income's taken into account for tax (if incomes declared) so will be taxed at 40% (some 45%) and a bit more if Scotland, plus any NI.
Theres no problem with people having 2 or more jobs, some of my team have more, but there paid for set hours or per hour. Mps should commit a set number of hours but of course its quality not quantity so maybe its up to the voters to decide whether theyve done enough.
I dont mind occasional second jobs as doctors or barristers like Kier but think all these lobbying consultants roles should be stopped