Numerous websites seem to suggest that EE has the fastest 4G network in the UK, followed by Three. However that difference might just be because EE has more masts, meaning that users are more likely to be very close to an EE mast than a Three one.
This site ran tests to check whether the speeds on Smarty are as fast as those on Three and found that there was no difference:
https://www.simsherpa.com/networks/compare/smarty-vs-three
Note this quote from that website though:
"Three are typically the second fastest 4G network, with speeds around 30.7 Mbps.
Those are the sorts of speeds you’d get on mid range broadband. However, in reality you can find yourself going as slow as 9 Mbps in rural areas or as high as 70 Mbps in more built-up areas"
As a reference point for you, I live in a small town that has notoriously poor mobile phone signals. (We used to have a mobile phone shop in the town which, somewhat embarrassingly for them, couldn't get a signal from any network). I've just checked the 4G speed I'm getting from the nearest Vodafone mast (which is used by my network operator, Asda Mobile) here in my bedroom several times. It's consistently around 1.5 Mbps. (The local press often report on communities here in Suffolk who struggle to get more than 0.3 Mbps, so there are certainly a lot of variations around the country).