I suspect the implementation of a minimum spend for free shipping was not only done in part to cover the cost of increasing shipping costs, but also to increase sales.
More people will be trying to fill up their basket to meet the £50 threshold, whereas before you could get away with just ordering a single B1G1F deal for £19.99 or a single £9.99 Blu-ray with free shipping.
Now, psychologically the customer is thinking I need to spend £30 more to get free shipping... I might as well add another B1G1F set and a few little things to get it over £50. This will no doubt increase the average value of their customer's baskets. It's a subtle form of upselling.
This will also help them logistically as they're not having to prepare and ship numerous small orders that are valued at £10-£40. Of course, there are negatives to this as you may turn away customers that only want to buy a few things, but as someone else mentioned, I don't think it's a coincidence they have implemented this shortly before the start of the Xmas sale where customers are more likely to indulge and place larger orders anyway.
With regards to the flash sales, we may just have to bite the bullet. If the prices are as good as last year, they'll still be the cheapest price available even with the addition of £3.50 postage if under £50.