Can I board a UK train I am booked on half way with a mobile, advanced ticket? [duplicate]

If it's an advance, then no, advance tickets need to follow the route exactly ("You may not start, break and resume, or end your journey at any intermediate station except to change to/from connecting trains as shown on the ticket(s) or other valid travel itinerary.").

From a practicality point of view, the train guard would almost certainly not know, but getting through ticket barriers at the station (which Birmingham New Street has) would be the issue, since you'd need to convince the guard to let you through (your ticket will not open the barrier as per normal)

This sort of change to a journey is allowed on most types of train ticket in the UK. Unfortunately, an advance ticket is an exception, and does not allow any changes at all; you have to board and leave at the stations specified, not in between. (Advance tickets are notorious for being particularly inflexible.)

However, you say that the journey is still some time in the future. It is often possible to request an exchange for your Advance ticket, updating the details to a different set of details; with cardboard tickets this would be done via handing in the tickets at the ticket office of any staffed station, but the process may be different or nonexistent for mobile tickets. (With mobile tickets in particular, the terms and conditions say "However, please note that if you choose (where offered) to print your tickets at home or download them to your mobile device, different rules apply and you may not be able to subsequently change your ticket."; it's nonetheless worth checking to see if a change is possible, because "may not be possible" doesn't imply "is never possible".) Note that there is a fee for the change in journey, normally £10 plus the difference in price; as such, it's a good idea to look into the possibility of an exchange as soon as possible, as the price is normally lower the earlier you book the ticket.

It's quite possible that (as you're changing to a shorter journey) the difference in price is less than minus £10, so (assuming that the exchange is technically possible) it may be possible to convince the ticket office staff to exchange the ticket for free.

You could just ask before going on the trip. Just walk to the nearest big train station to the reception if there is a possibility to do this in a legal way.

Sometimes they are more accommodating than one might think. I actually had a similar problem when I went to London because I bought the wrong (non refundable) ticket by accident, they did change it to the one I needed without any extra fee.

If that fails you can still try it the "sneaky" way. But most probably, asking won't hurt you.

There is a route between Southampton and Manchester which changes at Birmingham New Street (using the National Rail website, any of the services with one change appear to change at Birmingham). As others have said, you may need a different ticket to get through the barrier at Birmingham, but if your ticket allows for travel by Any Permitted Route, you'd have no problem using it to board at Birmingham.

Unfortunately, Advance Singles are usually for a specific service. If this is the case with yours, and the train you're booked on requires a change at Birmingham, then you should be fine to simply board the Birmingham leg as you would have to anyway. If your ticket doesn't change at Birmingham, then you throw yourself at the mercy of the conductor on the train. It's likely that he wouldn't notice or would use his discretion, but the risk is high enough that it's worth checking the cost of changing your ticket before making a decision.