I just though I'd post this list since people aften come here and ask about what visas are required for a nanny/au pair from abroad. These countries participate in the Working Holidaymaker scheme , which allows a young person (in his/her 20s I think?) to enter the UK for 2 years and work for 12 months.
And, obviously, EU citizens don't need any kind of visa.
Antigua and Barbuda
Australia
The Bahamas
Bangladesh
Barbados
Belize
Botswana
Brunei Darussalam
Canada
Cameroon
Dominica
Fiji Islands
The Gambia
Ghana
Grenada
Guyana
India
Jamaica
Kenya
Kiribati
Malawi
Malaysia
Maldives
Mauritius
Mozambique
Namibia
Nauru
New Zealand
Nigeria
Pakistan
Papua New Guinea
Saint Christopher and Nevis
Saint Lucia
Saint Vincent and the Grenadines
Seychelles
Sierra Leone
Singapore
Solomon Islands
South Africa
Sri Lanka
Swaziland
Tanzania, United Republic of
Tonga
Trinidad and Tobago
Tuvalu
Uganda
Vanuatu
Western Samoa
Zambia
Zimbabwe