On our Facebook group we sometimes get questions about how Brexit impacts those currently living 'under the radar' in France. Although RIFT admin condemns deliberate illegality and tax evasion in the strongest possible terms, and we adhere to the French principle that nul n'est censé ignorer la loi we realise that life is complicated and that some may find themselves in this position due to naivité or to changes in their situation that they maybe didn't think about.
If this is your situation, with Brexit looming, you really have some things to consider:
Note: its absolutely fine not to have a Carte de Séjour yet, that doesn't put you under the radar. This post refers to those who, through ignorance or dishonesty, haven't so far engaged with the French authorities, in particular by failing to declare their world-wide income.
Can you get yourself into the French tax system? You can often do a 3 year backdated declaration, as France lets you correct errors in your tax declaration for 3 years. Depending on your circumstances, you may need to settle up for tax owed, or you may find that tax you paid in the UK covers you, as the bilateral tax treaty is not affected by Brexit. Perhaps consider taking professional advice before doing this.
Healthcare. Unless you are here on holiday, you should not be using an EHIC for your healthcare. Once you have 3 months official residence, you can apply to join PUMa, the French Universal Healthcare system. If you are a UK pensioner it might not be too late to apply for an S1, though there is a backlog. Other than that, you really need to look into private insurance to bridge the gap until you qualify for PUMa, in case you get sick as medical costs mount up fast.
Carte de Séjour - We know that some on low incomes are worried about this, but at some point you are going to need to make a decision, as at some point, all British citizens living in France will almost certainly need one. To be covered by the WA, you will need to demonstrate legal residence before the end of the Transition Period, which brings us back to the need to file an income tax return. Be aware that the Préfectures have some discretion. For example, if you own your home, then there may be flexibility on the income requirements. If age, health and general circumstances allow, it might be worth looking for work (even low paid and part-time, as this will boost your income and sort out your healthcare situation.)
In general nothing is going to improve if you ignore it, and if you want to be covered by the WA you really need to document your presence in France before the end of the Transition Period. It might make sense for some to use the early months of 2020 to regularise their tax and health situation before applying for CdS.