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I am a solicitor.
You say that your international driving licence is valid until July 2020? Is that a mistake?
In that case, you are fine to use it for 12 months from August 2021 as your insurers in the UK have advised.
All the best
The rules state that you are permitted to drive for up to 12 months from when you became resident.
You were not continuously resident in the UK since 2018.
The main thing is that you are insured in the UK and you should declare to your insurers that you also hold a UK provisional licence.