No, that's not possible. You can only travel in your country of residence on 2 of your travel days. Those days will have been used after your first return trip to the UK.

What you could do is board Eurostar in Lille the second time and use other trains to get to Lille. That will cost a bit extra of course.

Please also note that Eurostar has a 30€ mandatory reservation + a passholder quota. Book well in advance, especially on weekends, holidays and thoughout summer.

If your dates are set far in advance (and you only wish to visit London) it might be cheaper to buy regular tickets. Of course if you need to go to Manchester or somewhere else a pass is likely good value!

Thanks for the help already.

I would have to travel to Swansea or Pembroke in Wales so further than London, that's why I was wondering if it's worth buying the pass.

I'll have a look at travelling from Lille also, thanks

TGV Brussels-Lille is often available for €19.

If you travel via Kortrijk or Tournai, you can use a ticket to the border and from there your pass will be valid. You can also use a Rail Pass to the last station in Belgium and a separate ticket from there to the border (should be €2.50).