The slide of sterling, like the slide of the dollar and the rise of the euro are due to very simple supply and demand economics.
Traditionally, many developing countries in the world kept foreign currency reserves of dollars and/or sterling. This is because for a lot of these countries, their own currencies are week or unstable. This created a demand for dollars and sterling beyond US & UK domestic use. Where there is demand for something, the price goes up. So the price of the dollar and sterling has been high for past decades due to this demand.
Obviously, on the date of the launch of the Euro, no country in the world had Euro reserves as the Euro did not previously exist. Once the Euro got bedded in and proven to be a very stable major developed economy currency, the aforementioned developing countries decided that it would be prudent to keep a large proportion of their foreign currency reserves in Euro. So recently they have started to sell their dollars and sterling and buy Euro. This has causes a glut of dollars and sterling on the world markets - thus the price decreases. And the extra demand for Euro has caused the price of the Euro to rise.
There is no reason to suspect that this trend will not continue for the foreseeable future as confidence in the Euro is still rising. Long term, sterling may end up being the biggest loser as some of these countries will decide to stay with only 2 main foreign currencies - Euro and dollars - and not bother with sterling which has been relegated by the Euro. Many of these countries are talking about heading for a 50:50 dollar to Euro ratio in their main reserves.
In addition to the above, the UK has the problem that they are an EU country that is not trading in the EU currency - the Euro. This erodes their competitive in the internal EU market as exchange rate risks and costs have to be built into the price of all UK goods.
In short, the future of sterling is not looking good.