When we have the final changeover, all tvs must be capable of receiving digital tv - either from a set top box, a built in digital receiver, through a cable provider like NTL, or Sky.
As long as your portable tv is connected to one of these, it will receive digital tv.
But what you have to remember is that unless you have a two tuner device, you will only recieve one channel at a time. So no recording another channel to watch later, as you do now.