I think your best bet in summer time is the dorms of various London Uni campasses – cheap, chearful, reasonable food and very security conscious. Here’s some to whet your appetite - Google accordingly:
http://www-mice.cs.ucl.ac.uk/multimedia/misc/local.html#westminster
The London School of Economics (Room B508, LSE, Houghton Street, WC2A 2AE) has a number of its halls available at Easter and summer.
Carr Saunders Hall (tel +44 (0)20 7323 9712), 18-24 Fitzroy St W1 (tube: Great Portland St), is not the greatest part of town, but it’s central enough; open Easter and summer holidays; singles/doubles with full breakfast are £17.50/30 over Easter, £22/42 in the summer. Self-catering flats of different sizes are also available on a weekly basis in the summer only, ranging from £238 for a two person flat to £539 for five persons.
High Holborn (tel +44 (0)20 7379 5589), 178 High Holborn WC1 (tube: Holborn), is centrally located and has self-catering singles/doubles for £25/43, with continental breakfast, in the summer only.
Passfield Hall (tel +44 (0)20 7387 3584), Endsleigh Place WC1 (tube: Euston), consists of ten late-Georgian houses in the heart of Bloomsbury. A bed and full breakfast is £17.50/30 at Easter, £19/35 in the summer.
Similar rates apply at Rosebery Avenue Hall (tel +44 (0)20 7278 3251), 90 Rosebery Avenue EC1 (tube: Angel).
Imperial College of Science & Technology (tel +44 (0)20 7589 5111), 15 Prince’s Gardens SW7 (tube: South Kensington), is in a brilliant position near the Kensington museums; open Easter and summer; B&B £25/40.
Regent College (tel +44 (0)20 7487 7483), Inner Circle, Regent’s Park NW1 (tube: Baker St), is a converted Regency manor right in the middle of beautiful Regent’s Park, convenient to Camden and central London; open May to September; singles/doubles £25/36.
John Adams Hall (tel +44 (0)20 7387 4086), 15-23 Endsleigh St WC1 (tube: Euston), is central and facilities include a swimming pool; open Easter and summer; B&B is £19 per night for more than seven nights, £21.40 for less.
King’s Campus Vacation Bureau (tel +44 (0)20 7351 6011), 552 Kings Rd SW10 0UA, administers bookings for a number of central King’s College residence halls. Rates range from £14 to £23 per person, include access to excellent facilities, and most include a continental breakfast.
Finsbury Residences (tel +44 (0)20 7477 8811), Bastwick St EC1 (tube: Old Street), comprises two modern halls belonging to City University. Available over Xmas, Easter and the summer, you get a bed and continental breakfast for £19.50 per person per night or £97.50 a week; evening meals are available on request.
University of Westminster (tel +44 (0)20 7911 5000), 35 Marylebone Rd NW1 (tube: Marylebone), has singles and doubles for £15.70 per person under 26, £21.50 over 26, and £3 for an optional continental breakfast.