My 18 year old will be travelling to London and staying for approx 1- 2 months max. She will then be travelling to Slovakia to stay for a few months on a National Visa (if the Visa is granted). She is only wanting to book a one way flight, as she does not yet know her exact return date. Will she have any problems at the London airport arriving with a one way ticket?

>>Will she have any problems at the London airport arriving with a one way ticket?<<

She very well might -- A young person arriving w/o return travel plans is a red flag. in fact they may not let her on the plane in the first place. The airline is responsible for returning any passengers who are denied entry so they will be careful to examine your daughter's itinerary.

She will 99% have trouble . My husband and I in our 70s were asked to show our return ticket from Uk to Australia when we were boarding the Eurostar in Brussels by UK immigration . "How do I know you will not use our health system , overstay etc"
Her age would certainly raise queries . We have had young friends sent back to Australia who haven't had return tickets .

She could buy a fully-refundable return ticket and then return it for a refund before it expires.

For that matter, she could buy a fully refundable ticket long before she leaves for London, and then cancel the ticket a day after buying it. There are no paper tickets anymore, so the only thing she'd need, presumably, would be a print-out of the confirmation of a flight that leaves the UK with the period of her permitted stay.

Before purchasing anything check out the price of a round trip ticket (with, say, a 30 day or longer return date) as against two individual one way tickets. In both cases take into consideration the $250-300 or so it would cost to change the return date and be sure the return portion is changeable.

Purchase whichever is cheaper and use the most probable planned return date so that you would only have to make a date change if the alternative plan comes to fruition.

Thank you very much- this was very helpful.

The UK is one of few countries in the world where immigration officers do ask questions to visitors. [The others I have been to are US, Canada and Australia.] So, be prepared to have answers ready, whether you're telling the truth or not.