Compared to France, the UK has a very long window for submitting tax returns - just under 10 months compared to ~2 months.  It can be tempting to wait until the last minute - however, that is usually a bad idea if any questions come up that need more research (such as digging into a specific pension and where it is taxed) or where your tax residency lies.

Certainly for French residents with UK obligations a good time is June - all paperwork from the UK should be ready, your memory is fresh from what you put on your French return but its all finished, and there is plenty of time to sort issues.  A classic will be if you have left the Uk and are letting out your old home - it can take a long time for HMRC to click the right buttons on this - and you may end up putting in a paper tax return and praying it does not get lost by HMRC.  Which based on some internet comments seems not uncommon.

Also, if you have a sudden realisation that you got something wrong with your French tax you can get in touch with Impots and fix it.  

So do it in the late Spring to Summer then you can file it safely online - and avoid any penalties.