I'm talking generally here.
In Rayners case its yet to be seen how this pans out with HMRC - likely she will get a penalty for 'careless' behaviour. However what did for her was the breach of the ministerial code.
As for Rayner being wealthy, that might soon be applicable in the past tense:
https://www.telegraph.co.uk/politic...-may-have-to-sell-hove-flat-after-losing-job/
Quote in case paywalled: admittedly there is some speculation but this does seem to stack up quite well.
"Angela Rayner could struggle to afford the mortgage on her £800,000 holiday home after having her salary heavily reduced, following her
resignation from government office.
Ms Rayner also faces a hefty tax bill and possible “carelessness” penalty of more than £50,000 for failing to pay the correct stamp duty on the purchase of a flat that sparked her downfall.
In a further blow, Tories demanded she be stripped of her £16,876 severance payment owing to loss of office because in opposition she had voted to stop ministers under investigation getting a pay-off.
“If she has any integrity then surely she must decline any severance payment,” said Kevin Hollinrake, the Tory party chairman.
Ms Rayner’s salary will be cut from £161,409 to £93,904 – a drop of £67,505 – giving her little margin for error in making her monthly mortgage repayments.
Earlier this week, she admitted she had used her life savings to put a deposit down on the flat. She sold her 25 per cent stake in the family home in Ashton-under-Lyne in Great Manchester for £162,500 and used that to put down a £150,000 deposit on the Hove property.
Angela Rayner may struggle to keep up with payments following the loss of her well-remunerated job
Official documents show she has a £650,000 mortgage on the seaside flat through NatWest.
The scale of the loan will have left her with mortgage repayments as high as £4,000 a month while her salary against an income of £5,400 a month after tax. As deputy prime minister she was taking home £8,100.
Following her resignation, she will have just £1,400 a month left over with two teenage children to look after, covering food and clothing bills, gas and electricity, holidays and sundry other costs.
She also has a £40,000 tax bill to pay as well as a likely penalty of £12,000 plus interest on top of about £1,000 – a total bill of £53,000.
HMRC will have to decide if she should pay the excess for ‘carelessness’ in punishment for originally paying just £30,000 in stamp duty rather than £70,000 on the Hove flat."