flight one way Porto to England only

[Explorer-8]

I am trying to plan a holiday whereby I travel by train from England to Paris; stay in Paris a few days; travel by aeroplane from Paris to Porto in Portugal; stay in Porto a few days; travel by aeroplane back to England.
The problem is that all the internet sites only allow bookings from England to Porto one way, not Porto to England one way. One said "cannot book return flight without outward".
Is there a way of booking a flight from Portugal to England one way only - cheap flights preferably?

[dzug]

Both Ryanair and Easyjet serve Porto and I'm sure allow a one way from Porto to the UK.

[Buenchico]

The charter airlines which serve Porto will only allow you to book return flights. But the scheduled airlines (Ryanair, easyJet, TAP and Luxair) should all allow you to book a single flight. (They usually show the fare in euros, rather than sterling, for such flights).

Perhaps you've forgotten to click the 'one way' button on the flight search page. (By default, airlines' websites normally assume that you're booking a return flight. You need to click 'one way' before entering your flight details; if you don't, you'll be told that you can't book the flight).

Chris