Hard to find concrete information, but try this elderly article (it's not dated, but clearly written between 2016 - 2021)
Some quotes:
Her Majesty’s Revenue and Customs (HMRC) was created in 2005 by the merger of the Inland Revenue and Her Majesty’s Customs and Excise. The newly formed HMRC had around 104,000 staff. In 2017 the headcount in HMRC stands at approximately 58,000, but under proposals ironically titled “Building Our Future” they intend to cut thousands more jobs by 2021.
HMRC’s budget after years of systematic cuts was 40% less in 2016 than in 2000.
For more than a decade HMRC has faced year on year cuts to funding at the same time as ongoing internal re-organisations. Together these have led to a situation where HMRC has been variously described as a government department in “crisis” and “not fit for purpose”.
Staff morale is so low that even in areas where HMRC is not seeking to cut jobs, experienced staff are leaving in droves, worn down by years of pointless “change management” and relocations that never lead to better efficiencies, either cost-wise or service-wise. This is set against a backdrop of a tax code of more than 16,000 pages, a tax gap of approximately £119bn (see below) and a complete dismantling of face- to-face services to the public.
www.taxjustice.uk/tax-takes-6.html
There was one in the financial Times that looked promising, but behind a pay wall. I'm going to have to bite the bullet and get a digital subscription. I think.
Because of cuts in staff and funding HMRC is consistently failing to collect £30 - 40 billion of tax each year. That's the government's own estimates. Check out 'The Tax Gap'