99% sure it's the same device - think it's hosted on a VPS at lcn.com.

Using the mattlantis.net proxy gave me a stable 19ms in game to froschroom which suggests it's not a froschroom related issue and is instead some BT weirdness.

Protocol would have involved about 77 udp packets per second on port 27501 - so yeah, different to a ping.

In a windows tracert I was getting around 130ms on the penultimate hop, but the final one was 19ms. If it happens again tonight I'll try a UDP traceroute and see if that makes any difference.

Tracert is of limited benefit as it just tells you the priority something gives to ping responses rather than the actual time. The only hop that matters is the last one in this case and that is giving you the same response as ping (as would be expected).

As I said before it is either shaping/prioritisation of the particular protocol or it is the fact that the servers take longer to return a response. The fact that the latency for the game is going to be based on what the server is doing and therefore measures the actual servers latency on dealing with the packet rather than just the latency involved in returning a ping could account for the difference.

I would also strongly suspect they have multiple back end servers being serviced by a load balancer. Or that there are front end servers for "presentation" and back end servers doing the application work. In either case it would be the front end/load balancer that responds to a ping but the back and app servers actually doing the work for the game.

The ping shows your raw latency to the server. The game shows the amount of time it takes for your packet to get from you to the server, to be processed by the server and then for it to be sent back. Do you know peope on the game with other ISPs that can compare their ping latency with the latency reported by the game at the times you are talking about? It could just be a busy time for the game.

Assuming though that others latency in game is fine then it could very easily be prioritisation by the ISP on that port/protocol. Then it would be interesting to see if you can find any other BT customers that can compare latency.