depends if you have the budjet for the branded ones or if it will get regular use, but theres nothing wrong with 3rd party products most are pretty good, just dont go too cheep, nissin do decent flash guns, but cannon do a smaller one for around 80-100 pound,
if your using it regualry go for a better one,if not go for a cheeper alternative,
With regards to the flash, one major question. What do you want to do with it? If you're thinking of getting it off the camera (which I would strongly suggest as it's the single main point of having an external flash) I would consider the ease of triggering it remotely.
With the 7D and a Canon own flash you will be able to fire everything remotely without the need of extra equipment and cables and you'll be able to maintain full TTL metering and everything, just like using the pop-up flash on the camera.
Using a third party flash you won't get any of that. You'll get TTL with the unit on camera but you won't get remote triggering without an extra cable or some sort of wireless system (either IR or radio). Wireless triggering systems start at about £40 for something simple like the Interfit Strobies.
Nothing wrong with decent 3rd party flashes though. SWBMO as a Nissin Di622 on a Canon EOS 60D and it's great. Works a treat. Just bear in mind that if you want to get the flash off the camera you'll need to invest more money, so your 3rd party flash for £50 will require another £50 (or more) to make it behave in the same way.
With battery grips, it's not quite the same. I've never used a 3rd party grip. I would suggest that it depends on which exact make - which one are you looking at? I'd have some concerns given that it's what carries power to the camera so you don't want it going wrong and frying your expensive 7D.