The rules on travelling in and out of various areas are guidance NOT law currently, even in and out of VERY HIGH alert areas. Current guidance says to ‘avoid’ this, but it is not illegal.
However, when area goes into Lockdown, another part of the Rule of Six applies, which is law, and that is that you can only travel in or out of locked down area as one household or one household bubble in England. This covers staying overnight and it’s illegal to mix with another couple. An example: an Airbnb, where the let rooms are part of the same house is now an illegal let in any lockdown area, as more than one household or household bubble is staying under one roof.