One way ticket, will they require me to have the other way booked?

You might have some problems as they want to know you are not trying to overstay. See this other recent thread for comments.

https://community.ricksteves.com/travel-forum/general-europe/one-way-flight-to-europe-from-u-s-is-that-okay

Isn't a one-way ticket considered a slight flag for "possible terrorist"? It might cause additional scrutiny that could delay your leaving the U.S. is my point. Aside from the European fear of job-stealing.

It will depend on your age, your demeanor at the immigration check, your answers, and the officer. What they are trying to prevent are someone who may overstay visa, may try to work a little under the table, etc. If you provide satisfactory answer, then you should have no problems. We have never been asked for a return but our son who was traveling by himself at 24 was asked.

At one time in the US the one way ticket received extra attention. Not sure that is true today since this is widely known.

Our daughter purchased a one way ticket to Granada in 2013 to take a 4 week Spanish immersion course. She had no return ticket and purchased her flight home after completing the course and touring around Spain and Portugal.

Of course, this was a few years ago.....

Well, your committed now, so all you can do is prepare. If it were me, I would do the following...

Have a solid itinerary for the UK. Where are you staying the first nights, where are you going from there, how long will you be in the UK?

Obviously you need to get from the UK to the Continent. How are you doing that? Flying? where's the ticket, Eurostar? Where's the ticket? Ferry?

If you have a plan and show how you are leaving the UK, then you will fare better. The UK is the toughest immigration routine I have encountered (Canada a close second) you have to have the answers.

You will have less scrutiny once you get to the Schengen countries, but there will be enforcement on exit if you have overstayed your welcome

As mentioned above, UK passport control is stricter than in some other countries. In my last several encounters with them, I've nearly always been asked for evidence of my flight home. I wouldn't recommend that anyone try to enter the UK without having booked firm plans for onward travel. You may get lucky and they'll let you through anyway. Or you may not.

Immigration going into the UK always asks very pointed questions...where are you going, how long are you staying, what date do you return home, why are you here, etc. Just have your answers ready ahead of time for anything they might ask, even if you don't have a solid plan. You need to say that your plans are firm. They'll usually only ask you for proof of a return ticket if they suspect you plan to overstay. They are becoming more strict all the time though and you don't want to risk being turned around right then and there and being put on a plane home. It has been known to happen.

Upon entering the UK at LHR, I have been asked the usual questions but not always " when are you going home?" For sure, I have never been asked at Border Control to prove it, ie, showing them the return electronic ticket. Most of the time I don't where the print out is, just somewhere in the luggage. Asked once about the B&B's address in London I had reserved, (that space I had left blank), I didn't have the print out or know the exact address, only told them that it's located in Kings Cross, and " I just know how to get there."

This is an extract from the current UK regulations concerning admittance as a visitor:

The applicant must satisfy the decision maker that they are a genuine
visitor. This means that the applicant:
(a) will leave the UK at the end of their visit; and
(b) will not live in the UK for extended periods through frequent or successive visits,
or make the UK their main home; and
(c) is genuinely seeking entry for a purpose that is permitted by the visitor routes; and
(d) will not undertake any prohibited activities ; and
(e) must have sufficient funds to cover all reasonable costs in relation to their visit
without working or accessing public funds. This includes the cost of the return or
onward journey, any costs relating to dependants, and the cost of planned activities
such as private medical treatment

(a) is met easiest by having an onward ticket, even if (e) implies only needing the money for the journey out of the UK

Yes and no.

You have gotten good information here.

A lot will have to do with your age. Someone in their 20's is more likely, in their eyes, to break the rules, overstay and look for work compared to someone over 60.

Before I had Registered Traveler, I was always asked about the reason I was there, where I was going, in detail, until I got to the part of when I was going home. Once I told them the date, they seemed happy.

Without a return ticket, you may be asked to prove how you would pay for it or for that measure how you are going to pay for your stay. They could ask you to show proof you have enough funds to support yourself while there. You don't have to have cash. If you have credit cards and a way to show a bank account with funds, that will help. If you can't show this, they may think you are planning to work to make the funds to get back.

We always fly to London on a one-way ticket because we use miles and book one way at a time. It has never been a problem at all. The immigration officer asks how long we will be in the U.K. And we answer that question honestly. Never been asked to show a return ticket. And usually we depart London for somewhere else---train to Paris, or plane to Barcelona in the most recent trips. But never asked about details or proof of those plans.

So I suggest you have a firm plan on how long you will be in the UK so you can answer that question. We have never been asked for anything more.

You don't need a "return ticket" because many people don't return from the UK. They might move on to the mainland or Ireland. They key is answering firmly how long your stay will be and where you will be going next. Even if you don't know for sure, have an answer so you can satisfy the agent if asked. They won't insist you have, say, a Eurostar ticket to France, but want to hear that you will be taking it (for example). Or say, I'll be here two weeks and then moving on to Italy. If the agent wanted to see proof of a ticket to Italy, your answer would be that you'll either fly a discount airline to take the train through France.

What you don't want to be is non-committal. That might arouse suspicion. And as noted by the others, your age, demeanor, time of day, mood of agent, color shirt you're wearing could all impact the type of questioning you get.

Based on the information provided in the posts here,why would you take the chance? The customs officers are trained to detect any variations in your body language and speech. Penny wise and pound foolish in these times.

I have traveled to Europe often for 20+ years now. I dont think I have ever been asked this question. Maybe once in the 90s. As to UK they seam to ask more questions.

There are more ways to travel than by plane. Simple answer is you are continuing your journey via train or bus out of the region. For the most part as I stay longer I buy a ticket to return once in Europe.

Rule of thumb for me is when I travel I am clean, dressed comfortably in a collard shirt and clean jeans. Be what you are a friendly tourist. Push comes to shove answer truthfully.

From the a EU page .... http://ec.europa.eu/immigration/do-i-need-a-visa/do-i-need-a-short-stay-visa_en .

What does a Schengen visa allow me to do?
With a Schengen visa you can travel to any or all of the 26 Schengen area countries for the period of validity of your visa, but for a maximum of 90 days in any 180 days period. You do not need an individual visa for each country, and you do not have to show your passport when crossing borders between countries within the Schengen area. However, at the area's external boundaries - the borders with non-Schengen area countries - all travel documents are carefully checked.
Other entry conditions to the Schengen area are:
You must be able to explain why you want to enter the Schengen area and to prove the conditions of your stay. You must also be able to show that you have sufficient financial means for the duration of the stay and for your return;
You must not be considered to be a threat to public policy, internal security, public health or the international relations of any of the Schengen area countries;
You must not have an alert issued in the Schengen Information System for the purposes of refusing entry.

In short the answer seams to be financial means to return / leave.