The way it has been done in T5 to some extent answers these concerns. Family groups have their own area in Zone A (so the North end by the tube/HEX exits). For everyone else, as you enter the self drop area there is a member of staff to do things like the visa check first. And if you get stuck on the machine there is a rover staff member, working along a bank of machines, who will help you. In reality if someone looks like they would have problems with the technology then Rover will be called over and gently help them, giving all the required chit-chat about the weather and how nice Málaga is at this time of year. So if you want the full on experience, just look sufficiently inept and help will be provided. If it's a quiet time then a Rover is likely to pop by anyway. [I don't think Rover is the right term here, but it is used elsewhere in T5, e.g. the person at the top of the first escalators in South Lounges trying to shoo Silvers up the next escalator.]

The layout at T3 means you can still use a staffed desk. At LGW, though they are not supposed to do this, they often switch off the self tagging machines if things are quiet so they can deploy staff elsewhere.