I don't know about your country, but where I'm from the theft Act 1968 seems to cover it pretty clearly, and I am most definitely Not A Lawyer:

The basic definition of theft is given at the start
1:1 'A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it; and “thief” and “steal” shall be construed accordingly.'

http://www.legislation.gov.uk/ukpga/1968/60/section/1

You might immediately say that this discounts the possibility of the original owner committing theft, but 5(1) partly defines 'belonging to another' as:

'Property shall be regarded as belonging to any person having possession or control of it, or having in it any proprietary right or interest (not being an equitable interest arising only from an agreement to transfer or grant an interest).'

http://www.legislation.gov.uk/ukpga/1968/60/section/5

So, the computer appears to 'belong to' the pawnbroker, as they have possession and control of it. But let's look back to 'appropriation'.

3(1) 'Any assumption by a person of the rights of an owner amounts to an appropriation, and this includes, where he has come by the property (innocently or not) without stealing it, any later assumption of a right to it by keeping or dealing with it as owner.'

http://www.legislation.gov.uk/ukpga/1968/60/section/3

So, regardless of who 'really' owns it, taking it from the pawnbroker (to whom it 'belongs' under the act) is 'appropriating property belonging to another' However, now we look at the definition of 'dishonest':

3(1)A person’s appropriation of property belonging to another is not to be regarded as dishonest—
(a)if he appropriates the property in the belief that he has in law the right to deprive the other of it, on behalf of himself or of a third person;...'

So, if the owner believes that they have a right to take it, then they can't be guilty of Theft. However, if the owner takes it without that belief, then they may be. I don't imagine that it would be hard to convince a jury or magistrate that that belief existed.

Perhaps other offences under the act are appropriate:

22(1) 'A person handles stolen goods if (otherwise than in the course of the stealing) knowing or believing them to be stolen goods he dishonestly receives the goods, or dishonestly undertakes or assists in their retention, removal, disposal or realisation by or for the benefit of another person, or if he arranges to do so.'

http://www.legislation.gov.uk/ukpga/1968/60/section/22

This would imply that if you take stuff believing it to be stolen (which the owner would have a hard time denying, as those are the grounds on which the repossession attempt is being made), then you are guilty of "Handling Stolen Goods"

However, looking further into the act, we see:

2(3) 'But no goods shall be regarded as having continued to be stolen goods after they have been restored to the person from whom they were stolen or to other lawful possession or custody, or after that person and any other person claiming through him have otherwise ceased as regards those goods to have any right to restitution in respect of the theft.'
http://www.legislation.gov.uk/ukpga/1968/60/section/24
Therefore, it must be impossible for the owner of an item to handle stolen goods.

Section 11 governs the offence: 'Removal of articles from places open to the public.'

11(1) '... references in this section to a collection do not apply to a collection made or exhibited for the purpose of effecting sales or other commercial dealings.'

11(3) 'A person does not commit an offence under this section if he believes that he has lawful authority for the removal of the thing in question or that he would have it if the person entitled to give it knew of the removal and the circumstances of it.'

http://www.legislation.gov.uk/ukpga/1968/60/section/11

So it can't be that, either.

The Anti-social Behaviour, Crime and Policing Act 2014 gives provision for 'Low value shoplifting', this item is less than £200 so would be treated as such. However, this is just about whether it should be dealt with by magistrate or crown court, and requires an offence under 1(1) of the Theft Act.

Do I get my D.Jur?