There are various dates available for Paris, Lille & Brussels for £53 return, there is good availability for September and October although I have not checked other months. You can also get a one way ticket to Amsterdam for £30, although you will need to arrange your own transport back (i.e either a Thalys HS train to Brussels and a Eurostar back to London or a flight), with this being due to the ongoing border arrangement difficulties at Amsterdam.
First check suitable dates on the Eurostar website, the date matrix (shown in the image below) is clear and concise. Next, to get the price advertised you need to book via the Loco2 website (no booking fees). Create an account first and confirm your email before adding your chosen ticket to your basket (it is messy doing this process in reverse). Then apply the code FIVELOCO2 at checkout (£5 off £25). You can of course just book through the Eurostar website, but the total cost will just be £5 higher