Hi there
For what it's worth, I did this for about 5 years in a row but found last year (Sept time frame) that these offers are now almost non-existent as the companies become wise to the practice.
Also depending on your age and size of mortgage, you may be asked to submit to a health check. Each year (as you get older) this check becomes more comprehensive and you are taking a risk of something coming up that means the insurance company turns you down/hikes up a premium. Also, I was advised that because of the equality legislation, females will no longer get cheaper life assurance and thus any savings up to this year may well be obliterated if looking to change next year when the equality legislation has taken effect.
In summary, found it brillaint for a few years but depending on the elements mentioned above, just be aware that there are considerations before taking this course of action. As it happens, we will no longer be looking to do this as the risks are too great/mortgage still very big/age factor really starting to kick in with regards to the health testing required.
Hope that helps
S