No return booked, options open.
Return options include:
Indirect flights via your city of choice.
Ferry to Helsinki in the morning for c.£30 (IIRC from
AFerry.co.uk) and a direct flight back to London for c.£100 or less.
Ferry takes a couple of hours. Sailings on Tallink Silja line on Monday morning at 7.30am and 11.00 am. There is also a Viking Line trip at 8.00 am but that is 30 minutes longer and requires you to book a cabin for some reason. Tallink Silja seems the most flexible and quickest..
I have a site I use to check flight options...you can"t book through it but it has loads of filters and stuff to narrow down your options.
It"s
ITA Software. Just enter your destination airport as "LON" so it checks all London airports (if that"s what you want). One note..to ensure that it calculates taxes and things correctly, click the "More options" button and enter "LON" in the Sales City box.
The "older" version of this site (which is in beta) is
HERE and offers a "matrix" for flights times/airports etc.