Hello there woodelf,
In our area (South Eastern Trains) you can use a cheap day return for any train arriving in London after 10.00 am.
However,as I discovered recently,many train operators seem to have different rules.
You need to ring the specific train operating company and ask them,or check with their website.
IF you need to ring the National Rail Enquiry Line,do so before 6pm when it comes up in the UK( they seem to know what they are doing)
I rang after 6 pm and got an overseas call centre,who gave me completely wrong information.