This is not a grammar question per se, but I have trouble understanding what they mean in the following:

Being an island, Britain had been traditionally reliant on maritime communications. It was for this reason that she had always maintained a large navy. In fact, Britain had the largest fleet in the world. Therefore, sea power during world war 2 was not significantly different to that of the first world war.

The last sentence is the one I have trouble with. I am not sure if sea power is referring to Britain's sea power (during WWII), or it is referring to sea powers of every nation which was involved in WWII. I tend to think it's the latter, but it would be nice if it was validated by someone else more enlightened.
It's only about Britain.
It's poor writing: there's no logical connection between the fact that Britain had the largest fleet in the world and that it did not change between the wars. It could have significantly changed (in plus or minus) but still remained the largest.
The meaning of the sentence is that there was no important change between the two wars.
Also, I prefer significantly different from, not signficantly different to. See the BBC:

370 from bbc.co.uk for "significantly different from"
201 from bbc.co.uk for "significantly different to"
Since you brought it up...

Here is an interesting article about different from/to/than:
Teachers: We supply a list of EFL job vacancies
Thanks Marius
Students: We have free audio pronunciation exercises.