You will not easily get 10Mbps upload for £25 inc line rental unless you choose a provider with known congestion issues where upload is likely to drop down significantly.
Best deal you will find will be around £10 per month + line rental (on a deal for x amount of months before rising).
To keep 20 Mbps upload you will need to get 76Mbps services which will be more costly.
Are you out of contract with Plusnet or not? If not, my advice, wait for the contracts last month and negotiate then.
Edited by ukhardy07 (Thu 07-Jul-16 12:59:11)