What is Flashpoint and why's it so significant?

When I look at story arcs, it has a list of the issues it appears in, but it confuses me because you don't need them all. Which ones do I actually need?

I'm a UKer, where's the best and cheapest place to get comics?

Please don't just send me links to other places on the site, I've looked and don't understand. Thank you

Flashpoint was pretty much DC's way of saying 'Hey, we can totally reboot the universe like we've done over and over before but this time make it different kind of somehow maybe." When it was over, the three major universes of DC were fused together into one Frankenstein amalgam of realities. And just like every time before, one, maybe more, person/people knows what's happened in case DC decides it's not working out.

Flashpoint is an event comic which ushered in the DC relaunch known as The New 52. The only thing you have to read from Flashpoint to understand it, is the main issues, Flashpoint 1-5. The other issues involved in the story arc are just tie-in comics that help you get a bigger view of the different characters that are involved in the event.