Best answer by woz
Good reply from Stephen and to add he has a couple of options to trace who the supplier is, look HERE, but as said PP will know and deal with them anyway so even if he can't find out it doesn't matter as far as the switch is concerned. If he does find out he should be able to contact them and ask what the property has used previously.
However the much easier options are as said to use the OFGEM Values HERE tweaked to reflect his situation a bit higher than the low for gas if it has lot of outside walls or it's top floor, lower if sandwiched by other flats. A bit higher for electric if more people and an electric shower. (Also depends on how big the flat is)
If he doesn't want to get caught out I'd go a bit higher than the low figures he will only pay for what he uses and as we are moving into the cold weather he can always keep an eye on the statement balance and tweak the DD after 6 months.
The other alternative is to find a neighbour in a similar flat and ask. (good way to make friends too...or not...)
You'll find that if he keeps a note of the meter readings he will know in about a month what the electric usage is going to be as it tends not to vary much.
My son has just moved into a new flat so I'm considering suggesting he use pure as his energy supplier. His previous flat was electric only and his new one has electric and gas. He's only just moved in so hasn't identified who currently supplies the new home but as he won't have any idea of the likely usage in the new flat how will that work when changing suppliers to pure?