(I can't lay hands on it at the moment but there are clear guidelines to UK judges on what constitutes reasonable fines for offences, such that it should be feasible for the person to actually pay the fine)
Not true.
https://www.legislation.gov.uk/ukpga/1971/38/section/25
> The fourth, fifth and sixth columns show respectively the punishments which may be imposed on a person convicted of the offence in the way specified in relation thereto in the third column (that is to say, summarily or on indictment) according to whether the controlled drug in relation to which the offence was committed was a Class A drug, a Class B drug or a Class C drug; and
https://www.legislation.gov.uk/ukpga/1971/38/schedule/4
Cannabis is currently class B, thus
> [F8 3 months or [F4 £2,500], or both].
The courts must follow the sentencing council guidelines unless it's in the public interest not to do so.
https://www.sentencingcouncil.org.uk/wp-content/uploads/Drug...
The starting point is 100% of weekly income; the range is 75% to 125% of weekly income.
> Band B 100% of relevant weekly income 75–125% of relevant weekly income
https://www.sentencingcouncil.org.uk/about-us/
> The primary role of the Council is to issue guidelines on sentencing which the courts must follow unless it is in the interests of justice not to do so.
> The Sentencing Council is an independent, non-departmental public body of the Ministry of Justice and replaced the Sentencing Guidelines Council and the Sentencing Advisory Panel in April 2010.