Speed test

How did you measure this? Were you connected with an ethernet cable to the sky router?

That really isn't a reliable test. Until you have measured it directly on a wired connection you can't say for sure. Too many variables with WiFi like were you connecting using a 2.4GHz connection or 5GHz?

You can get your connection speed from the Sky Hub statistics page, one of which will be your connection speed. This can be different to your actual download speed. See this guide here:

https://helpforum.sky.com/t5/Broadband-Talk/How-to-read-your-Sky-Hub-statistics/ba-p/3645346

Also, you should read up on the Sky Speed Guarantee (i.e. your contract if you have concerns).

https://www.sky.com/help/articles/sky-fibre-speed-guarantee

As for third party speed test websites, Ookla's is probably the most popular. However, I prefer M-Labs, as it is the cleanest interface and lightweight (no ads).

Always run a speed test with no other apps running (including backround apps where possible) and have your browser set to private/incognito mode.

Those stats only work for xDSL connections, OP would be on a FTTH package if it’s supposed to be 500. The router at most will report the negotiated Ethernet speed to the ONT (likely 1Gb).