Thought this was an interesting one. You would think you come across this one regularly.  
 
Are transfers between unmarried couples done at market value?
 
s.17 TCGA 1992 states:
 
17  Disposals and acquisitions treated as made at market value
   
 
17(1)  Subject to the provisions of this Act, a person's acquisition or disposal of an asset shall for the purposes of this Act be deemed to be for a consideration equal to the market value of the asset–
 
(a) where he acquires or, as the case may be, disposes of the asset otherwise than by way of a bargain made at arm's length, and in particular where he acquires or disposes of it by way of gift or on a transfer into settlement by a settlor or by way of distribution from a company in respect of shares in the company, or  
 
(b) where he acquires or, as the case may be, disposes of the asset wholly or partly for a consideration that cannot be valued, or in connection with his own or another's loss of office or employment or diminution of emoluments, or otherwise in consideration for or recognition of his or another's services or past services in any office or employment or of any other service rendered or to be rendered by him or another.  
 
17(2)  Subsection (1) shall not apply to the acquisition of an asset if–
 
(a) there is no corresponding disposal of it, and  
 
(b) there is no consideration in money or money's worth or the consideration is of an amount or value lower than the market value of the asset.
 
My conclusion is therefore that s.17(1)(a) as the transfer is arguable not at arms length, and could be deemed to be a gift or partly a gift HMRC may try to argue along these lines.
 
IR CGT Manual states:
 
CG14532 - Consideration for disposal: market value rule applies
TCGA92/S17 & TCGA92/S18
YOU USE THE MARKET VALUE OF THE ASSET INSTEAD OF THE ACTUAL CONSIDERATION WHICH PASSED BETWEEN THE PARTIES WHERE
 
 
 
THE TRANSACTION IS OTHERWISE THAN BY WAY OF A BARGAIN AT ARM'S LENGTH, in particular?  
 
a gift  
 
a transfer into settlement by a settlor  
 
a distribution of assets by a company to its shareholders  
 
the asset is disposed of wholly or partly for a consideration which cannot be valued, or  
in connection with the loss of office or employment or reduction of earnings to any person, see CG16270+, or  
 
in consideration for or in recognition of the services of any person  
 
THE DISPOSAL AND ACQUISITION OF THE ASSET IS BETWEEN `CONNECTED PERSONS' (defined in CG14580+)  
 
WHERE A DISPOSAL IS DEEMED TO TAKE PLACE AT MARKET VALUE,  
 
for example
where a beneficiary becomes absolutely entitled to property as against a trustee where, in the hands of trustees, assets are deemed to be disposed of and immediately reacquired on the termination of a life interest.  
 
For guidance on the operation of the market value rule where assets are acquired or disposed of on exercise of an option, see CG12395+
 
So a pure gift would be caught.  
 
What if not a pure gift but a transfer at an under  
 
 
Where there is an exchange of assets this is a bit clearer:
 
IR CGT manual on exchange of assets:
 
CG13090 - Exchanges of assets: introduction
 
These instructions tell you what happens when assets are exchanged between parties and where consideration is not always paid, see CG13092. The exchange of one asset for another is a chargeable occasion for both parties involved.  
 
There are, however, special rules which apply in cases involving
 
the conversion of securities, see CG55000+  
certain exchanges of shares or debentures, see CG52520+  
the transfer of a business to a company in exchange for shares, see CG65700+  
the exchange of joint interests in land, see CG73000+.
 
And finally IR manual in definition of arms length:
 
CG14541 - Consideration for disposal: market value rule: at arm's length
 
A bargain made at arm's length is a normal commercial transaction between two or more persons. All of the parties involved will be trying to obtain the best deal for themselves in their particular circumstances.  
 
This does not mean that a bad bargain cannot be a bargain made at arm's length. For example Mr A may wish to sell his property quickly so that he can go and live in Malta. Mr B knows that Mr A wants to sell his property quickly so he offers him a low price for a quick sale. No-one else makes an offer. Mr A accepts the price Mr B has offered. This may not have been the best possible price which Mr A could have achieved if he had left the property on the market for longer but he was still trying to achieve the best deal possible for himself. It was a bargain made at arm's length.  
 
Another example where a bad bargain could nonetheless be a bargain made at arm's length is where one party to the transaction has better information about the asset than another. For example Mrs S may sell a picture from her attic to Mr T for £500. Mr T, who is an art dealer, knows that the picture is worth £5,000. There has been a bargain with both people trying to get the best deal for themselves. Again, this is a bargain made at arm's length even if the price paid is not the `market value' of the asset.
 
 
Boyfriend or girlsfriend type transactions and other "unconnected" person transactions could be caught. Was just checking this out on a case I am on and though might be helpful on here !
 
Lucy