I am told the 'no deal' Brexit plan is that your EASA licence will be automatically converted to a UK ATPL on 29 March, which will allow you to fly G-reg aircraft. The position on M-reg etc (other states of registration are available) is not quite so clear.
You can also expect a lift and shift of the Basic Regs and the rest of the EASA-related hard and soft law into UK law, effective for 2 years. The intent is that the UK will mirror any EASA changes in this period too. At least, that is what the DfT is saying.