I've been having the exact same issue as you, also based in Brighton, BN2 area.
The service has been working great all week for me, however it all fell down yesterday at about 5-6pm.
I'm on a rolling monthly contract, M350 package.
Getting 380Mbit download, 0.3 - 0.1 Mbit upload when testing via a variety of services, including speedtest.net, speed.ui.com, fast.com
I checked the Service Status page and it tells me there's no issues, however when I use their "issue checker" it tells me they're working on fixing an issue. But the Service Status does not reflect this.
As well as trying the “check service,” Area status webpage" (link at top right of this forum), also try the “free & automated” Service Status number - 0800 561 0061 - which usually tells you of more local issues down to postcode level (that may not be listed on the Area Status web page which usually only covers >1000’s of customers) - that may give you more info, but even that wont cover problems affecting just a few customers.
-------------------- John --------------------
I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router +2 Airport Express's. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.