Hi Olaf, I'm new to the Dis but saw your question and thought I would clarify some of the previous answers.
The UK is a union of countries just as the US is a union of States. England and Wales have one legal system. Scotland has another. Northern Ireland has another (albeit similar to the legal system in England & Wales).
The biggest differences in the legal systems are between the Scottish and the others. It would be like comparing the Louisiana legal system with that of New York State. Whereas the Northern Irish and English systems are like the differences between New York State and Massachussetts.
In the English legal system (which covers Wales too), the procedure for purchasing a property is to make an offer SUBJECT TO CONTRACT. The subject to contract bit allows for legal searches etc. to be completed before a contract is actually concluded. This is important because as a purchaser you want to ensure that the person selling the property to you actually owns it; that the property has not been illegally built; that the property is not about to collapse; that a new freeway is not about to be built next to the property etc.
So...the procedure is:-
1/. You make an offer (this is the telephone bid you saw on the TV programme) but it is not binding until contracts are exchanged.
2/. You exchange contracts. This can be the same day as the bid but is more likely to be 1 month later.
3/. Completion. This is the transfer of ownership date specified in the contract when the purchaser pays the money and the seller gives vacant possession. This could be the same day but is more likely to be one month later.
At point 1/. a purchaser might have to pay a holding deposit to the realtor as a sign of good faith (say $500-$1000) but not strictly necessary.
At point 2/. a purchaser will have to pay a deposit. This is normally 10% of the cost but might be reduced to 5% by mutual agreement. If the purchaser pulls out subsequent to this he will lose his deposit. Additionally, the seller can still sue the purchaser to force the sale to happen. For this reason a purchaser should for example insure the property he is purchasing from exchange of contracts because if the property were to burn down before completion the seller could still force him to pay the agreed price for what is then a smouldering wreck.
At point 3/. the purchaser has to pay the balance outstanding (90% or 95%) to the seller.
Consequently, whilst it could happen in an extreme scenario where someone is looking to exchange contracts and complete on the same day that a purchaser could simply pull out, it is an extremely unlikely event.
What is common is for purchasers to pull out prior to exchange of contracts, even on the day that this is due to happen. This could be because for example they found somewhere else instead, or they found the property needs say a new roof or other substantial repairs which they had not realised.
This is further compounded because it is not usually one transaction but a series of transactions: A buying from B, who is buying from C, who is buying from D, who is buying from E etc.
This is called "a chain". The more properties in the chain the greater the risk of failure. To prevent B from being contractually bound to sell his property to A before C is contractually bound to sell to him etc. the attorneys ("solicitors") acting for all the parties will aim to exchange contracts simultaneously - i.e. all the parties in the chain have to exchange together. This is why it frequently goes awry because a problem anywhere in the chain can hold up the entire chain.
I hope this explains our system to you.