If your valuation for mortgage purposes was based on £150k, then the property is hardly valued at £120,000. There is too much difference between those figures, and creditors will be very suspicious.
I do not think that they will ask for the extra £8k as a modification, as there is more risk that they will see you do not need an IVA at all.
I think you now need to ask your IP their opinion as to the merits of your offer, as presumably they would not wish to put an offer forward they were not going to get accepted. Do be careful about those property valuations though. Your IP will need to have a valuation of £120k if that is what he/she is relying on in the proposal, and creditors are bound to figure out there was a higher one for re-mortgage purposes, given that this is where you are sourcing the settlement monies from.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
Appoint me as your IP:
http://www.melaniegiles.com/ivaEnquiry.asp