Maybe a bargaining ploy, I’d be surprised if HMRC will take any less than that are owed.
At one time and over the years, there was an implication that HMRC would accept a compromise if the likely proceeds of any admistration meant they would receive a lesser amount than that owed in the final distribution of realised assets.
However, since December last year, HMRC have been elevated to secondary preferential creditors in any administration. That means they rank behind Creditors with a fixed charge, Adminstrators Fees/expenses, and Prefential creditors (money owed but unpaid to employees),
but ahead of prescribed creditors (ringfenced fund), secured creditors with a floating charge (banks), non preferential creditors (suppliers) and shareholders (Morris etc).
Unless there is something fundamentally flawed in their claim. there is no way HMRC will dilute their legitmate right to unpaid taxes and contributions with their new position. That would create a precedent and undermine their efforts over many years to raise their status in insolvency proceedings.
If you asking me how I know, it's because I've been there many moons ago in a previous life, but long before they finally got their status elevation.