I would think it's harder to get the journey cheaper than £30.
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Using Condition 19c :
19. Using a combination of tickets
You may use two or more tickets for one journey as long as together they cover the entire
journey and one of the following applies:
(a) they are both Zonal Tickets (unless special conditions prohibit their use);
(b) the train you are in calls at a station where you change from one
ticket to another; or
(c) one of the tickets is a Season Ticket (which for this purpose does not include
Season Tickets or travel passes issued on behalf of a passenger transport
executive or local authority) or a leisure travel pass, and the other ticket(s) is/are not.
Here's my option for you :
London Euston - Milton Keynes Central VIRGIN ONLY SDR £7.25
Milton Keynes Central - Rugby ANY PERMITTED SDR £3.50
West Midlands Day Ranger £5.40 (Priv Discount is available on this).
Total : £16.15
The West Midlands day ranger is classified as a leisure travel pass. I think i'm correct with the above option satisfying condition 19c ( will someone please clarify).
And also be prepared to have a discussion with the Barrier Staff at Euston.