Hi,
Any equity release is based on the value of the property as it stands i.e. 480 k - bank of scotland will lend up to 95% of this,
to answer your other question, there is nothing to stop you having a different lender for your private home and investment, however the bank would be more in favour of giving a loan for investment if they have the private home, also if there is a shortfall they will look at the bigger picture. (that they are getting both loans) Also an advantage of using one lender is, that in some circumstances the lender will take income for the private home and rental income for the investment as long as the rental is approx 1.4 times the interest only amount. if there is a shortfall then the lender will take the whole package into account.
I hope this helps