Well it wouldn't be a temporary Septic Tank but a temporary Cess Pool/cesspit. There is a difference as I explained in another thread on Septic tanks.
An IBC only holds a thousand litres and you would fill it with normal useage in a few days. The average household uses 500 litres of water a day. Pumping out by a contractor would probably be in excess of £100 a time (if they would do it as it does not comply to any regulations). It is illegal for anyone other than a licensed contractor to dispose of the effluent.
Personally other than as in a dire emergency I don't think it's a great idea.