Any hotels cost less than HK$1500 in London?

You can visit www.expedia.co.uk or www.zuji.com.hk. The former site sometimes offer very good last minute offers but have to pay in GBP.

I do not suggest staying in hotels (especially 3-person rooms) with less than GBP60 per night per room. The Millenium Gloucester Hotel, near the Gloucester Road Station, is always my favourite hotel.

你可以試下用priceline上網bid, 我到係用hk$1,000就book到holiday inn kensington forum, 近Gloucester Road Station (in Kensington area). 其它人用GBP60都係book到B&B ja.
不過唯一缺點無早餐, 要出街食.
你地一家人細, 都係住hotel好d, 唔好貪平住hostel 啦.

You can try Travelodge: http://www.travelodge.co.uk/. If you have booked early enough, you may be able to find 26 pounds per night around London. Some travel lodge may offers family room, which has two single bed and a sofa bed (can be converted to double bed) and can fit 4 people.

You can try to book "Easy hotel" or easyhotel.com. They provided very cheap price hotel. And they have 3 location in London.
I have booking on Mid Aug. just 40GBP per night.