Im a lurker of these posts but I thought i would provide some proper advise to this OP.
First of all if you have rats or mice on your property, then under the Prevention of Damage by Pests Act 1949, under section 3 it states that there is an obligation of occupiers of land to notify local authority of rats and mice if it comes to their knowledge that rats or mice are living on or resorting to the land in substantial number . Any person who fails to give a notice which he is required to give under this shall be liable to a summary conviction fine. So you need to report this and get it dealt with usual with pest control.
Secondly do not under any circumstances catch any rats or mice for any reason, despite the usual issues with disease. This will put you in place a duty of care for the welfare of all animals under human control (even on a temporary basis), under the Animal Welfare Act 2006. Which means you are fully responsible for its welfare and or dispatch if there is justified reason only in a humane way.
Under The Wild Mammals Protection Act 1996 requires that in the context of pest control, animals should be killed in a ‘reasonably swift and humane manner’.
The following methods are considered to be humane:
• Destruction of the brain by a strong and accurate blow to the head with a suitable implement
• Lethal overdose of appropriate gaseous or injectable anaesthetic (Not available for general use)
• Destruction of the brain by shooting
By administering Ibuprofen to the rat is not legally classed as humane and there for illegal. The second issue now is that if any other animal consumes the corpse of an illegally poisoned animal, it may poison that animal also (E.G pets, protected species, endangered birds of prey) which could lead to conviction and further suffering of other animals.
As for releasing it in a field may bring you into disrepute with the land owner as you are transferring a known Pest with would come under the Prevention of Damage by Pests Act if they found out and could land you in trouble.
You should have left the rat where it was and called a professional pest control officer. Being ignorant of the law is not an excuse if you were to be reported.