Beware of scams and cheats claiming to be UK companies or institutes, or even pretending to be the High Commission. The cheats behind these could actually be anywhere in the world. There are many popular tricks and ploys used by fraudsters, but in the most common examples they often issue an email to inform victims that:

a)  they have won a cash prize in a lucky draw
b)  a parcel has been received for them and is being held en route from some country
c)  they have an 'unexpected' inheritance that they need to collect
d)  they need to remit or transfer money to designated bank accounts in order to exonerate themselves from alleged crimes or an underpayment of tax

In most scams, victims will have to make a fee payment in advance to facilitate the transfer of the prize money; the administrative or processing work, through a remittance company or to a bank account. Victims are requested to provide their personal particulars and a bank account number.
 
Please do not make any advance payments or reveal your bank details if you suspect a scam.

Here are some telling signs:
 
1) Scammers using a free email account (e.g. gmail, hotmail, etc.)
2) Terrible spelling, punctuation and syntax. Grammatical mistakes
3) Incorrect addresses and other details, including false names
4) Scammers tend to use mobile (cell) phones rather than land lines. UK mobile phones start with 07 (or, when called from overseas, 00 44 7)

Remember, if you do not personally know someone it is very unlikely that they would contact you and offer your something valuable.

Useful links:
 
HM Revenue & Customs related scam examples and information click here
Metropolitan Police Service's fraud alert
Consumer Direct: Scams - what to look out for 
More details, including how to report a scam, can be found here