The 'A' in 'ADSL' stands for 'asymmetric'. In order to get your phone line to work in the best way for most internet use (which involves downloading quite a lot but uploading hardly anything other than 'page requests') the system deliberately allocates over 90% of your line's capacity to downloads, rather to uploads. (For example, I'm currently getting 9.96Mbps for downloads but only 0.96Mbps for uploads:
http://www.speedtest.net ).
Further, a 25 minute video (using the high definition that many cameras, phones and tablets now offer) use a massive amount of data. (Perhaps 5Gb if recorded on an iPhone but far more if recorded on an HD camera).
Check your download & upload speeds (using my link above) and see how good they are. Here are some figures (for conventional copper-wired phone lines) for guidance. (I've quoted download speeds. Roughly divide by 10 for the corresponding upload speeds):
The Government's target download speed for all homes is 2Mbps but many rural areas still get less than 1Mbps.
The minimum usable download speed for streaming services (such as Youtube, BBC iPlayer, Netflix, etc) is typically around 2.5Mbps.
Many people (without fibre optic cable services) who live reasonably close to their local telephone exchange get around 3 to 5Mbps as their download speed. (Most people in my street, which is about 2 minutes walk from the exchange get around 5Mbps).
People who get an 'ADSL 2+' service (rather than the usual 'ADSL Max' that BT wholesales) get roughly twice the speeds of their neighbours (on 'ADSL Max'). So, because I pay to have ADSL 2+, I get around 10Mbps.
The fastest download speeds though copper cables tend to be around 12Mbps. (Slightly faster is theoretically possible but, it seems, rarely ever achieved).
See how your speed compares with those figures. If it's broadly in line with them (and preferably near to the upper end) it's unlikely that a different ISP could offer higher speeds unless
(a) they can offer a fire-optic cable service (which would be your best option to achieve a higher speed) ; or
(b) they can upgrade you from 'ADSL Max' to 'ADSL 2+' (which will roughly double your speeds.