去london最便宜的B&B

Normally they give you the cheapest price ga, but since you are going in end of May (near summer holiday), the price is more expensive! If you want to stay in B&B, on average it's about £20-30 per person per night (children is 12 years old or below). Anyway, if you want to have a quick look at all the hotels or B&B, you can go to this website:
www.lastminute.com
From there, click on HOTEL, and choose Location as LONDON. They give you fix price for that day and won't change it. You can also book it there if you want.

Option 2 is, you can consider staying in a Aparthotel (短期出租屋), where you can fit all 4 people in, and you can even cook in it!! Try the below address:
www.central-london-apartments.com

Option 3, you can try to see the price at Easyhotel (英國其中一間廉價航空旗下既一間廉價酒店). The website is www.easyhotel.com

Option 4, you can contact the local chinese travel agencies in London. They may do promotions with some hotels. Also, they speak Chinese, so it's easier to bargain. Some of the chinese agencies that I know are:
泗和旅遊: www.seewootravel.co.uk/index.php?hotels
歐亞旅遊: www.europe-travels.net/Others/promotion07frame.htm
歐美嘉: www.omegatravel.net/cn/shop/hotelsearch.html

Hope it helps!