You can try Aikido first, before buying any special clothing - wear a T-shirt and tracksuit bottoms for the first few sessions. Some of the classes offer a deal on your first few lessons (eg. 4 lessons for £10 instead of £4 per lesson). Once you have decided Aikido is for you, you will need to buy a gi (cost £25.00 including club badge) and join the British Aikido Association (BAA). Membership of the British Aikido Association costs £24.00 per annum.
You just need to remember one single thing right from the very first day of your Aikido career.... Enjoy yourself !!