In the UK, area code 01303 (or international +44 1303) is Folkestone.
An international number beginning with +1 303 (dialed as 00 1 303 from many places) is Denver, Colorado, USA.
(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, and many other countries use different prefixes.)
Country code +44 is the United Kingdom. Area code +44 1303 (or 01303 in UK domestic format) is Folkestone.
All UK area codes begin with a 0. There is no area code 6010 in the UK, and there is no 06010 code either.
UK area code 020 is London.
There is no 01134 code in the UK.
There is no area code 20 in the UK
The country code and area code of Aberdeen, United Kingdom is 44, (0)1224.
In the UK, 0121 is the area code for Birmingham.
Cambridge is in area code 01223, or +44 1223 from outside the UK.
Until the early 1990's, area code +44 21 (or 021 within the UK) was Birmingham, which is now area code +44 121 (or 0121 within the UK). There has never been an area code +44 217 (or 0217) in the UK.
The area code "029" is assigned to the city Cardiff in Wales, UK.
Slough. There is no 0753 code in the UK.
There is no UK area code like 0491. If the number is pre-1995 then this area code is now 01491 which covers Henley-on-Thames.