As the others have explained, your line is connecting as fast as it can go.
Infinity 1 is supplied using the Openreach 55/10 product. It is advertised by BT as 52/10 because of the Advertising Standards Authority rule that at least 10% of an ISP's customers must be able to connect at the advertised speed. It isn't capped at 52Mbps, you are just one of the 10% who can exceed it.
That also explains a BT fiddle of their statistics
, that I don't know of any other ISP using. They will not supply Infinity on any line where the estimate is below 15Mbps. That means that anything slower is excluded from the calculation, making it easier for them to attain the 10% requirement.
It is a cheat of course. Though less so than it used to be. They used to supply the same 55/10 for these lower estimates, calling it "Faster Broadband with Fibre" instead of "Infinity". AIUI for these below 15Mbps lines they now provide "Faster Broadband" using the more recently introduced Openreach 18/2 product.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. 200GB. Sync 67851/12343Kbps @ 600m. BQMs - IPv4