Prices are lowest during novemeber and december generally and then pick up again in the new year. As said above VAT is likely to have very little impact on second hand cars as it’s only paid by the dealer on the margin.
The dealers I’ve known have all varied there sales target dates so as to not end up with lots of people waiting for the last week of the month to buy and try and get a better deal.
In terms of where to buy if you know anyone who’s knowledgeable about cars then I’d be tempted to go private. If not then a trade sale will get you some kind of warranty but you could also purchase a car through a private sale and then buy yourself a warranty from someone like warrantydirect (who had a good rep last time I checked).