Speeds have by all operators always, ALWAYS been touted as 'up to' speeds, there's no guarantee there of 10Mbit.
Unless an ISP owns the entire internet they cannot guarantee speeds to anyone. Even leased lines costing a few hundred quid a month only guarantee bandwidth inside the service provider's own network.
If you want 'guaranteed' 10Mbit best dip hand a lot lot deeper into pocket than £35/month
Bulldog, UKOnline, Be, Telewest, all 'up to' speeds.