This is very similar to another post from a while ago. Seems they should really be asking a consulate or the state department.

They should be reading the Border Agency info, link in previous post.

Not sure the airline will allow you to board on a one way ticket, as they will be fined if you are deported.

Gardyloo definitely doesn't fit the demographic where return tix and boyfriend issues apply ;)

You really should have a R-T ticket - no matter what a consulate tells you, the final decisions rest first with the airline letting you on the flight in the first place, and the specific border agent you face at LHR.

Even if you are not young - boyfriend and no return date are enough red flags to at cause significant delay/questioning. And lying to them will only make things worse.

Get a R-T ticket . . . Even if you don't end up using it.

I feel like I'm in that Monty Python skit about the argument clinic.

I can only go on evidence that I know personally; I am not an immigration counselor nor do I have any special pipeline to HM border officers. However I do know that (a) my nephew, age 22, traveled to and through Britain last year on a one-way ticket; (b) another nephew (19) did the same the year before that; and (c) so did my daughter-in-law's brother (22) last year. I know this because I used my own frequent flyer miles to book them one-way tickets. Now maybe they are exceptions and maybe the border people have tightened up since, or maybe they don't fit some profile that triggers more scrutiny... I don't know. Obviously the OP should do the best she (assuming) could do to know before going.

There are as many different circumstances as there are passengers, and age alone isn't a decider when it comes to any potential difficulties at UK border. Itinerary, modes of travel, purpose of stay, ties at home all play their part. Maybe there was nothing in your relatives' profile to arouse suspicions, but visiting a boyfriend, with no return date (other than 6 months maximum) and no return ticket will present the immigration officer with a whole set of questions for which answers have to be found.