I'm a little biased, but wanted to let you know that Rhino Seeds (link in my sig) is reputable - and we're based in the UK.

Our homepage has a seedfinder that should help you find the right seed.
I filled in a few things for you and came up with Super Iced Grapefruit by Feminised Seeds and Night Queen by Dutch Passion.

Super Iced Grapefruit is a Diesel/Chronic mix with 650 gr/m2 yields (if you do it just right) and high THC levels.
It's recommended for indoor use, it's beginner friendly and it takes about 8 to 9 weeks of flowering.
The great thing about this one is that it's absolutely guaranteed - no questions asked.
With Feminised Seeds, they'll replace your seeds if they don't work out - even if you screwed it up.

Dutch Passion is one of the most popular breeders out there. They're Night Queen is an Indica with high THC & CBD with 500 gr/m2 yields (again, if you do it right). It's a little more old-school than Grapefruit, but still billed as beginner friendly.

Of course, automatics are always popular with beginners, but you have to be careful to get one that has a good yield and THC levels.

If you need any help narrowing it down, let me know and I'll see if I can help - by the way, check out our Twitter page - we give away seeds on a regular basis as no-strings-attached promotions meaning you don't have to buy anything to win.
I try to post all our free offers under our section of the forum.