There are few bus companies that can take you to/from Central London:
National Express: Most frequent service, prices start from £5 one way if you book in advance. It has a few stops to/from central London. Website: www.nationalexpress.com
Easy Bus: Probably the cheapest option, starting from £2 if you book in advance. It has a few stops to/from central London. Please note that EasyBus has partnered with National Express coaches, so basically compare the prices between the EasyBus and National Express before buying as essentially you will be in the same coach. The website is www.easybus.co.uk
Green Line: It picks up passangers from the same stops as the other 2 companies. One way ticket is £10 or £17 for an open return journey. Website: www.greenline.co.uk
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