It is very disappointing to have slow internet speed than advertised. We usually found slower internet speed compare to advertised by broadband provider. In this case you have some legal rights but its depends on the actual reason for getting lower download speed.
Broadband Speed is slower than advertised.
Broadband speed you're getting is not as you promised by your ISP. Your broadband download speed and broadband upload speeds should be match with the promised made at the time of sale, for example, guaranteed speed of 10 mbps will become a part of contract and if it's slower than it is a breach of contract. It may be a breach of contract. This may rise a claim against your internet service provider.
It is your right to claim for misrepresentation against your broadband provider for serviced offered at time of sign-up.
Advertised broadband speed not guaranteed
Before making any claim you should thoroughly check sign-up form and this may well say that the advertised internet speed is not guaranteed and is the maximum speed you may expect. Many internet providers use a word upto, which means you may get the speed but not guaranteed.
Your contract may even warn of factors that could affect the speed you receive, such as the distance from the telephone exchange.
Use a broadband speed checker
You can’t measure your broadband connection speed based on how fast it seems during normal use, as websites won’t necessarily make use of your full download speeds. You should use a HTML Speed test checking websites to see whether you are getting a significantly slower service than you should be able to get.
Check that all these things are working normally and are not contributing factors before complaining to your broadband provider.