Don't use BA (actual BA iron) since you will pay a fuel surcharge. 50k will basically get you on BA from MIL to BOS (via LON or MAD on Iberia) but you will end up paying close to $500 in taxes and surcharges.

You may be better off transferring the UR to United and getting a ticket on United. You can buy the remaining miles from Chase to get to the 60k

If you can find availability on Aer Lingus using BA miles, that would be your best option. Very low surcharges and you would get the RT for under 50 k including the connection from Milan to the UK