Lord Grey had been approved to raise a regiment in Leicestershire at the beginning of July 1648, at a time when county militias were beginning to be organised as a defence against Royalist uprisings (Commons Journal, v. 620). Grey was granted leave from the Commons to raise troops, as Members could not sit and hold military commissions simultaneously. One week after Grey received the Commons' approval, an invading Scottish army crossed the border in support of King Charles.
Grey's letter was addressed to Philip Skippon, then Major General of military forces in London. Skippon's command expanded in July 1648 in the light of Royalist risings in Surrey, Essex and Kent, and his authority was extended to forces beyond the boundaries of London. A religious Independent, the extension of Skippon's authority was loudly complained of by the more religiously conservative Presbyterians who formed the backbone of political moderation in the capital (Commons Journal, v. 622, 648, 677). London's Presbyterian-dominated Common Council had already called for a treaty with the King, a move that was endorsed by the House of Lords.
Skippon was himself the author of three religious tracts, devotional works aimed at his fellow soldiers in which he referred to himself as the “Christian Centurion”. Grey's letter, shot through with religious allusion, was clearly intended to chime with his fellow Parliamentarian's religious feelings.
The publication of “Old English Blood” should be considered an endorsement of radicalism (embodied by the old soldier Skippon) in the face of growing political moderation in the City and at Westminster, at a time when Lord Grey's political voice was muted due to his involvement in raising and commanding Leicestershire forces.
Firth, C. H. “Skippon, Philip (d.1660)”, in: Oxford Dictionary of National Biography (Oxford: Oxford University Press, 1885-1900)
Groby, Thomas Grey, Baron of Old English Blood Boyling Afresh in Leicestershire Men (London: 1648). Early English Books Online [online] available https://quod.lib.umich.edu/e/eebo/A85691.0001.001?view=toc
Journal of the House of Commons: Volume 5, 1646-1648 (London, 1802)
Plant, D. "Timeline 1648", in: BCW Project [online] available http://bcw-project.org/timelines/1648