Before you contact your ISP to complain about your internet connection, there are some easy steps you can take before hand, and if there is an issue, will give you amunition to throw at your ISP.
i) The original equipment your provider will send you, although is mostly cheap and nasty (yes, and even the ranted about BT Home Hub), your ISP will not accept any test results made with any other equipment. So make sure any test results you give to your ISP are made using the original equipment.
ii) Saying that, the quality of the original equipment has much to be desired. If you replace the original equipment with one of your own, shop bought systems, you are likely to get an improvement on your network reliability and speeds. Personally I tend to spend about £100 on a wireless router, but £50 is a good starting point, be sure to read any reviews before you buy, the internet is full of them!
iii) If you have an Android phone, there is a lovely app called WiFi Analyser (not sure if there is one for the iPhone, but would expect so). Your wireless station works on channels between 1 and 14. WiFi Analyser will look at all the wireless signals within range and give you the channels and strengths of each signal. Try and make sure you are on the least congested channel for best performance. Most wireless stations are configured to choose the channel automatically, so if you find your wireless is on a congested channel, easiest option is to just restart the wireless station.
iv) The best way to get to the internet is via a cabled connection. The cabled connection wins head over heals against wireless. Being a cable strait to the internet, you do not have worries about the performance and stability of a wireless connection, and the speeds (up to 1Gbit compared to 54Mbit of G Wireless) are alot more consistent. If you are having issues with disconnection, test your internet using a cable strait into the router. Any high definition streaming should definately be done with a cable. Cabled connections are also alot more secure than a wireless connection.
v) Most of the ADSL providers within the UK rent their network space from BT (Virgin Media works in a different way), so ultimately, most providers are under the mercy of BT with regards to Quality and Network capacity. BT however, have released a fault diagnostic site, which will test your line against configured and available speeds, and give back a reliable result. Before you run this test, you need to make sure the computer you are testing with is connected to the internet with a cable, wireless connected machines are not accepted for test results. All you need is the telephone number attached to the line, Java installed, and this website: http://speedtester.bt.com/
Just follow the instructions on screen, and take screen shots of any results it gives you. Your screenshots will be your ammunition against the ISP if it returns any faults. This test needs to be done with the original equipment they send you.
vi) If you want to monitor your speeds, and handy site is http://www.speedtest.net/ . It will test the speed of your connection, and give you an idea of what you are getting.
vii) Remember, cabled connections are best over wireless, any day. A good way to have a cabled connection, but dont want cables running every where, is to buy powerline networking kits. These will allow you to use your main electrical circuit as a network cable. PCWorld do them from here http://www.pcworld.co.uk/gbuk/powerline-home-networking/727_7090_70097_xx_xx/xx-criteria.html. The only thing I have connected onto my wireless is the laptops, everything else (PC, Network storage, HTPC etc) is connected with a cable, but that is just me)
viii) What most people dont realise, is your wireless will only work as well as the slowest connection on the wireless network. Although for most people, the internet speed is slower than an averable G wireless connection, this can show more the faster your internet speed gets. This means if you have a friend or family member sitting in the back garden, enjoying a beer and listening to youtube on the edge of your wireless signal, your speed inside the house close to the wireless station will match. Not much you can do about this im afraid.