Green’s Dictionary of Slang

partake of His/Her Majesty’s hospitality v.

[joc. var. on SE at His/Her Majesty’s pleasure]

to spend time in prison.

[UK]Strand Mag. Sept. 296/2: Partaking of Her Majesty’s hospitality, in a building specially erected for gentlemen who need a compulsory holiday.
[UK]F.D. Sharpe Sharpe of Flying Squad 157: One awful thug I know, who is now enjoying His Majesty’s hospitality, beat his woman up regularly.