In the UK then there are many miles earning cards for many different airlines so mentioning BA or Avios would definitely help get more specific advice.
If you want to fly a specific route I would definitely check BA reward availability on that route before taking the plunge with a card. I have found it virtually impossible to find 4 club world seats in the summer holidays anywhere in the US, ended up switching to Virgin. However, if you only need 1 or 2 seats and can travel off peak then you'd probably have better luck!
The Amex prg is a great starter card, 22,000 points if you're referred and double points on supermarket, petrol, travel and foreign spend. Also fee free for the first year.
The Amex plat gives 35,000 points but costs £450 a year, it does have many benefits for this fee. The good thing with these cards is that you earn MR points which you can convert to BA at a rate of 1:1 but if you can't find BA availability you can transfer to other airlines.
BA has 2 amex cards, the fee free one with 4,000 Avios and a 241 voucher on spending £20,000 and the premium card costing £150 with 19,000 miles when referred and a 241 on spending £10000.
There is a sticky at the top of the Amex forum where you can get a referral from people like me, already holding the card.People referring usually post the benefits with the card so it's a good place to get a quick overview without wading through lots and lots of threads.
http://www.flyertalk.com/forum/ameri...posting-2.html