I want to know if I am entitled to medical assistance in the UK while visiting for 3 weeks in June. I am travelling on my Canadian passport since my UK one is outdated.
The NHS is a residence-based healthcare system
Hospital treatment is free to 'ordinary residents' of the UK. But if you are visiting the UK – to stay with family, on business, as a tourist, or if you are living here without proper permission – then you are likely to be charged by an NHS hospital for the treatment you receive
As such, since you're a Canadian resident, you're not entitled to NHS coverage through the normal route. Citizenship doesn't help, it's the residency (and hence paying for it through taxes) that counts
There are quite a few countries with reciprocal healthcare agreements, under which residents of those countries can use the NHS while visiting, and UK residents can use their system when registering there. Unfortunately, Canada is not one of them.
As a British citizen, you're entitled to move to the UK. If you did, you could register with a GP, and start accessing NHS treatments. However, if you're just visiting, and don't qualify for one of the exemptions on this page, you'll need to take out travel insurance with a health component / arrange for your existing health insurance at home to cover you abroad.