There are at least two things you can do:
- Google for a list of EFL schools in London using key words and phrases such as Learn English in London, EFL school London, English school London, Teach English in London, etc. then email them all and ask them about flexible teaching hours. Practically all schools of that kind use part-time teachers.
- Google for a list of TEFL/TESOL job sites using key words and phrases such as Teach English London, TESOL job London, English teacher London, etc.
As always when using search engines, it pays to be imaginative. If you are not getting the results you expect, refine your search criteria.