Lockdown is making loneliness feel 1000x worse whatever the reason for it (I'm single and have no family nearby, myself).
However, in time after lockdown, now your son is 14 would he be able to supervise your daughter for a couple of hours a week so you can start pursuing a hobby you enjoy, say an art class, book group or sport? There may be some they could come along to such as walking groups that don't cost too much.
I agree, in the kindest way with PPs who say that for the time being (unless a solicitor said differently) moving back to Poland is off the cards, and your ex is settled in new relationship. I know it's easily said but these two routes aren't your only chances of happiness, you need to start looking at what can be done to improve your life. This will probably be small steps. What about regular zooms with family or friends in Poland, it might be a nice way to keep the kids immersed in their Polish culture?
Work-wise, a change can be as good as a rest so it could be a good time to have a look at jobs with a different retailer, maybe even on promotion.
In terms of your current neighbours, I know you say it's not the nicest area but what is stopping you from chatting to them briefly? Are there issues such as antisocial behaviour or drugs on their part or does the area itself just feel a bit intimidating? If there's nothing in particular, why not have a quick, casual chat to people living nearby when you see them? not with a view to being inseparable friends straight away, necessarily, just to have a few familiar and friendly faces around.
Online dating is draining even without kids. By all means give it a go but try and compartmentalise. That is to say only give it as much time and energy as you can comfortably spare and no more.