Our devices use the industry standard PPTP protocol to connect back to our VPN servers in the UK

If the PPTP session is not initiating for some reason, check with your ISP or local provider to see if they are blocking PPTP for some reason.

Your firewall should allow outbound connections using GRE (protocol 47) and TCP port 1723

If they are inadvertently blocking PPTP then talk to them about removing the block