I do not know if you are still considering a Three 4G connection, but there is no evidence to suggest mast
IV0104 even has 4G transmissions on Three, or actually more specifically anything other than LTE Band 20 (800 MHz, which Three "Super-Voice" uses... fancy term for VoLTE). Three's own checker shows 4G coverage for you only with 4G Super-Voice selected, otherwise nothing (well, besides 3G). Ofcom's mobile coverage checker (
here) shows 4G data coverage for you but I imagine it is using 800 MHz unless, for some reason, Three's own coverage checker is somehow outdated or this is wrong.
Now, you can use Band 20 for data but be warned that you will not get speeds anywhere near that of what Band 3 (1800 MHz) can offer you. This is partly down to the fact Three is only using 5 MHz as opposed to 15 MHz with Band 3 (ignoring the fact it is a lower frequency). In my experience, even selecting 3G (which I presume is 2100 MHz, UTRA Band 1) on my B525 outperforms LTE Band 20 with double the downstream (the upstream is a bit less, but no more than about half). Using an external anetnna on the device, on LTE Band 3 I get anywhere from (DS/US) 45-65/16-20 Mbps mostly, with LTE Band 20 about 5-7/2-4 Mbps, and UTRA Band 1 about 12-18/2-3 Mbps.
I could be completely wrong about all this and just talking out of my arse, or my I am just an outlier when it comes to my experience. But I am literally just going on all the information I could source. In the case that this information is correct, there is a good chance you are better off using 3G UTRA Band 1 over 4G LTE Band 20. Only testing will tell you what is available and how well it works.