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Of British-English origin, the phrase reasonable force designates the level of physical force deemed appropriate or justifiable by law:
– in protecting oneself or one’s family or property;
– in apprehending a criminal suspect or preventing a crime.

This phrase occurs, for example, in the following from Couple held over shooting ‘lived in fear’ of burglars, by Martin Evans and Nick Britten, published in The Daily Telegraph (London, England) of Wednesday 5^th September 2012 [page 2, column 2]:

A businessman arrested for allegedly shooting two burglars was “living in fear” after police failed to catch thieves who had broken into his secluded cottage three times previously, according to his family.
Andy Ferrie and his wife Tracey were released last night after being held for almost three days on suspicion of causing grievous bodily harm […].
[…]
The couple have been supported by Alan Duncan, their local MP and a Government minister, who said they were the real victims and it would be a crime if they were prosecuted for defending their home. Earlier this year, ministers said they would clarify the law to make it clear that anyone who uses reasonable force to repel an intruder will not be punished.

—Cf. also the phrase an Englishman’s home is his castle.

These are, in chronological order, the earliest occurrences of the phrase reasonable force that I have found:

1-: From an unsigned letter to the Editor, published in The Statesman (London, England) of Thursday 26^th October 1809 [page 4, column 2]—the “unfortunate old man” was J. Barrow, “a feeble man, of the persuasion of a Quaker, of threescore years and ten”:

The title of the cause was, The King v. J. Barrow, and the indictment for an assault on the Overseer, Constable, and Tax-gatherer, of Standish […].
[…]
The Counsel for this unfortunate old man […] insisted, that […] Barrow had only exercised his right, as an Englishman, to order any person out of his house whose company he did not choose; and by all reasonable force, without injury, to compel him, if he refused.

2-: From The Sun (London, England) of Tuesday 11^th March 1817 [page 1, column 3]—George Hanger (1751-1824), 4^th Baron Coleraine, was a British-Army officer and author:

Lord Coleraine.—Yesterday Lord Coleraine, better known by the name of Colonel Hanger, charged a hackney-coachman, at the Bow-street Office, with an assault, and with demanding more than his fare. The coachman, on being refused his excessive demand, became very insolent; and when Lord Coleraine proceeded, with reasonable force, to turn him out, he seized an oak stick from his Lordship’s hands, and struck him several blows about the head and face. He was fined for his extortion, and committed for the assault.

3-: From The Morning Herald (London, England) of Thursday 24^th March 1825 [page 2, column 4]:

It would seem, from what passed the other day at the Mansion House, that some of the members of the Stock Exchange are in the habit of becoming bulls and bears in more senses than one. Yet this place, we believe, is frequented by the select part of the fraternity; there being another and a lower grade, who are not allowed the entree of this bear-garden. We are not sorry that the matter has at length acquired publicity; and we hope, that if a public and ample apology is not made to the person who appears to have received such brutal treatment, that the offenders will be brought before a competent tribunal, and, if not put in the stocks, at least be made to pay a handsome dividend for their audacity. The instance alluded to is by no means a solitary one. It is the custom and usage of the place to treat every stranger who enters the doors with ignominy and violence. It cannot for a moment be contended that even where a person is an intruder, and has no right to be, he can be legally kicked and buffeted by the owner. The law recognises only a right to use reasonable force upon such occasions.

4-: From an account of a court case, published in The Morning Chronicle (London, England) of Saturday 20^th October 1827 [page 4, column 4]:

Assuming this room to be the Magistrate’s room, any person who chose to disobey his order to leave it, subjected himself to the unpleasantness of being turned out. The prosecutor owns that he disobeyed such order, and therefore subjected himself to the consequences. The assault, therefore, which he received, I maintain to have been an excusable or justifiable assault; just as if a person should choose to remain in the house of any private individual after the master of that house has ordered him out, the master would be justified in using all reasonable force in order to expel him.