Travelling to the UK without nailing down an onward journey is a poor idea.
Having an entry clearance basically implies that you have successfully completed your landing interview before you even arrived in the UK. Paragraph 25A of the rules makes this clear...
An entry clearance which satisfies the requirements set out in article
3 of the Immigration (Leave to Enter and Remain) Order 2000 will have
effect as leave to enter the United Kingdom. The requirements are that
the entry clearance must specify the purpose for which the holder
wishes to enter the United Kingdom and should be endorsed with the
conditions to which it is subject or wish a statement that it has
effect as indefinite leave to enter the United Kingdom. The holder of
such an entry clearance will not require leave to enter on arrival in
the United Kingdom and, for the purposes of these Rules, will be
treated as a person who has arrived in the United Kingdom with leave
to enter the United Kingdom which is in force but which was given to
him before his arrival.
It means they already know you and all your details and you have been "cleared" to enter the UK without further ado. So your landing interview will be shorter and more structured than for say, Americans.
HOWEVER, they need to check your documents for forgeries and alterations; and they are entitled to ask about your plans for this particular visit (if they want to).
Evidence of an onward journey is also important. While a return ticket
is no guarantee that a passenger will leave the UK, a single ticket
will often trigger further questions. If passengers have a return
ticket that is open, or dated after the time they claim to be leaving
the country, this casts doubt on their true intentions.
Source: Home Office Report: Exploring the decision making of Immigration Officers: a research study examining non-EEA passenger stops and refusals at UK ports
What they are saying is that if they want to see your onward journey and you do not have one, you will be in trouble, despite having an entry clearance. In the current era and public mood, it means your entry clearance will be cancelled and the IO will issue removal instructions. It is not a guaranteed outcome, nothing is guaranteed where an IO's discretionary latitude is concerned, but it is playing with fire, ill-advised, and wildly adventurous.
For completeness, I have to mention that it's avoidable if you are known as a high net worth individual (HNWI) or a known public figure or if your passport is wallpapered with previous entry clearances etc etc. In those cases the odds are improved.