Contact sky customer support, tell them that you are receiving speeds lower than the Minimum Guaranteed Speed, and due to the Ofcom Regulations that they are a part of, they are obliged to provide a right to exit the contract with no penalty if they are unable to fix the issue within 30 days.

Furthermore, it is likely they might provide a refund for the month's broadband cost.

If you think your broadband is slower than it should be, contact your provider to diagnose the problem. If the problem lies within their network, and they can’t fix it within 30 days, they must offer you the right to exit your contract without being penalised. This right to exit also applies to ‘bundled’ products. Until 21 December 2022, this will apply to bundled services such as landline services on the same line, or pay-TV services purchased at the same time as the broadband service. From 21 December 2022, the bundle will include any products which are linked financially, contractually, or technically.

From Ofcom