If you are talking about uploading, Plusnet have NEVER throttled or shaped uploads - the main limit will be the connection speed which is always much lower than download.
What sort of connection are you on (fibre or ADSL)? What are your sync speeds? Have you done a speed comparison direct as against over the VPN? What Plusnet account type are you on?
connection: ADSL, negotiated up/down rates are 1,219 / 20,847
scp transfer over vpn: 26.5KB
scp transfer direct: 144.7KB/s
account type: 'Unlimited' Business Broadband + phone