Actually the requirements are not too onerous and there are a few get-outs.
Under the Control of Pesticides Regulations 1986 (COPR),
operators must have had relevant training and be competent for
the duties they are to perform. Operators born after 31 December
1964 or who will be spraying as a commercial service must hold
the appropriate Certificate of Competence (see Table 1) issued by
the National Proficiency Tests Council (NPTC) or Scottish Skills
Testing Service (SSTS), unless they will be under the direct and
personal supervision of a person who holds such a certificate. If
born before 31 December 1964 and working on their own or their
employer’s land, operators still need to be able to demonstrate
competence and show proof of appropriate training.
More info here.
http://www.hse.gov.uk/pubns/afag202.pdf It also looks like anyone born before 31 December 1964 is less formally regulated for some reason.