You should check the details for the particular auctioneer that you are looking at but it is usual for them to offer indemnity insurance (to cover you/them if the car is stolen or still on HP) and on more modern vehicles they may offer the buyer the option to return the vehicle within an hour of the sale. Some auctioneers will state, as part of the description, if a vehicle has been an insurance write off but it's still worth looking for kinks in the chassis or doors/panels that don't quite match up. You shouldn't really have any problems with a vehicle on a 57 plate but, if you want to use the warantee, one of the conditions is usually that you have to have the car serviced regularly at a main dealer. What you might save through the warantee, you usually lose through the main dealer service charges.
There may be some genuine car sourcing companies but a lot of them are dodgy. A typical scam is for you to say that you want a 57 plate Jag, the sourcing company pass this information to a struggling business who buys a 57 plate Jag on a finance agreement then, without telling the finance company or settling the agreement, they "sell" the vehicle to you. The business then continues with the payments and, in effect, get a loan together with the responsibility to make monthly payments to the finance company. That could all work out fine as long as you don't want to sell the vehicle before the agreement is paid off and as long as the business can manage to keep up the payments. Unfortunately, this type of finance is often a last resort for a struggling business and, if the business goes into liquidation without paying off the finance agreement, then the finance company will want to take the vehicle back.
Other car sourcing companies simply take the money and run.
My opinion is that, if you are paying £30k for a car, you should, at the very least, kick the tyres before you get your wallet out.