Is it safe to use my multimeter to test UK wall outlet voltage?
Yes it is generally safe to use a multimeter to check the voltage on a wall outlet, but use caution on cheaper meters.

The meter will be employing a high resistance when on voltmeter mode (1 MOhm to 100 MOhm), so little actual current will flow.[1]

However, on ammeter mode, the meter will be employing very low resistance[2] and will cause a short circuit if connected to both live and neutral or ground[3]. It would also be very unsafe to touch due to the very high current. In the UK, wall outlets generally supply ~30Amps[4]

When I plugged my meter, on AC voltmeter mode, into an outlet's live and neutral, as well as live and ground, both times I received a reading of 239V on the screen. I did not test on ammeter mode as I did not have a proper circuit to test with in series.

Thank you all for your comments on my original post. Things weren't making sense to me until I wrote here and had your feedback.