Having problems with Cheltenham Gloucester mortgages

TAB1981

I have 2 buy to let mortgages with Cheltenham & Gloucester both over 20 year terms , these have been on interest only for the last 18 months since I got them , 2 months ago I swapped them to repayment but ive realised it would of been the best option for me if I kept them on interest only but overpaid on them. I rang Cheltenham & Gloucester today and explained that I would like to swap them back to interest only and I was told I cannot do this due to changes , the only way they would do this for me is if I proved I had the money in my bank or savings to prove I could pay them off straight away if I needed to ??? I wasn't happy over this and asked why I the customer was not informed of this when I rang to change the mortgages over surely they should of told me over the phone that id not be able to swap back to interest only once id changed to repayment , im waiting for the complaints team to contact me over this