Hiya,
as i understand it the MX record is not in IP form. you would point the mx record to xxx.co.uk
also you may want to consider setting up an a record that points the fqdn (ie servername.xxx.co.uk) to your fixed ip and then point your mx record to the fqdn. this will assist with ssl as the default certificate created on your sme will be fqdn.
cheers, shell