I can't answer precisely without knowing the full details of your journey and ticketing.
Leaving station premises, even when changing trains, constitutes a break of journey*. Some tickets allow break of journey, and others don't. Advance tickets in particular (those that have to be bought in advance and are only valid on one particular train) don't allow a break of journey; there are also some more flexible tickets that forbid it too, though most flexible tickets allow it. Break of journey can also be restricted sometimes by your route, if you do it at a station you're only allowed to be at thanks to certain routeing rules for instance.
Breaking your journey when you're not entitled to do so may lead to you being required to pay the difference between your advance fare and one that would have been valid but allowed break of journey, which could be quite substantial:
16.4. If you start, break or resume your journey at an intermediate station where you are
not entitled to do so, you will be liable to pay an excess fare. The price for this will
be the difference between the amount paid for the Ticket you hold and the lowest price
Ticket available for immediate travel that would have entitled you to start, break or
resume your journey at the station concerned.
(National Rail Conditions of Travel).
However, having said all this, it is very likely that even if your ticket doesn't allow break of journey, they will very likely take pity on you if you explain your situation to the gateline staff. In all likelihood if the station has barriers you will be allowed out (bearing in mind that using station facilities outside the barriers, if they exist, does NOT constitute a break of journey), and if it doesn't, in all likelihood there will be nobody there to stop you (even if they perform a revenue blockade they will likely not be doing this at the entrance to station premises, but the entrance to platforms instead — people will need to be able to enter the station to buy tickets after all!). I did, however, want to write an answer which explains what the actual rules say.
* The National Rail Conditions of Travel, since they were "simplified", no longer contains a definition of Break of Journey, but it's generally assumed that the old definition from the Conditions of Carriage likely still applies.