Many thanks nemeth,
It does seem then that once hundreds of new, subsidised, users take up this offer the Avanti satellite will follow Tooway and Eutelsat and become congested. I'm sure that if I had paid full price for hardware, connection and monthly packages I would be less than pleased.
It transpires that the only satellite being used for this subsidised "pilot scheme" is Hylas 2 - so the decision for me as to whether or not to take it up has been made. That satellite is not visible from where the dish would need to be sited on my property. I'm sure I won't be the only one with this problem, so until this "pilot scheme" is expanded to make use of other satellites, myself and others will need to make do with a 512k connection.
The advantage with H2 is that it's relatively under-subscribed in the UK. We use Hylas 1 for commercial services, and rent capacity by mbit for our userbase, and Avanti won't sell us Hylas 2 capacity. They have pretty much kept all of it for big contracts and big resellers, Avonline use H2 most places and usually only use H1 where they can't get LoS on H2.
Tooway/Eutelsat made the mistake of selling "unlimited" services, and this, I believe, is what killed their capacity as many people jumped on it. It also offered 20mbit/6mbit in a time when many were using 16mbit ADSL2+, so though satellite could actually be better.
Avanti sell two main services on H1 to resellers (you can't buy a connection from Avanti as an end user usually), "SVNO" (shared virtual network operator) whereby the reseller pays a list price per connection and resells it at a profit, with a set usage allowance, and avanti run traffic management etc. It's basically a white label service.
They then do "GVNO" (guaranteed virtual network operator) whereby the reseller buys bandwidth in mbits (usually around £1200 pcm per mbit) on each beam, then resells that to as many users as they choose, sets their own traffic management if any, etc.
Given that each GVNO is partitioned off from each other, even if one operator reaches saturation it wouldn't affect others even on the same beam. Avanti not allocating enough bandwidth to their SVNO platform could be an issue I guess, but I'm not sure there is an SVNO facility on Hylas 2.
They also do other services aimed more at commerical, blue light, etc clients, such as more high end hardware rather than the Hughes stuff (iDirect for example, where in theory you could get ~40mbit down from an X7 modem).