I don't think it matters when you cancel it - you always end up paying for the days you had the boost.
My understanding (which of course could be wrong) is:
Let's say the boost costs £15 a month with a one month minimum term and you take it out on 1 January. Let's say your billing date is on 15th of the month.
Your bill on January 15 has a charge for £7.50 for the period 1 January - 15 January and a charge for £15 for 16 January - 15 February.
The system won't let you cancel it until January 31 (maybe February 1, but I'll assume January 31 for now). Let's say you cancel on 31 January.
Your 15 February bill has a refund of £7.50 for the period 1 February - 15 February. You paid for that on your January bill but in the end you cancelled the boost so TalkTalk give the money back.
So, short term you over-pay as there is an assumption you will be keeping the boost, but if you don't you are refunded on the next bill.
Had you cancelled on 1 February rather than 31 January you would have paid for one day more - the refund on the 15 February bill would have been a bit less than £7.50.
One slight optimization is to buy a boost early in the day and cancel it late in a day as you pay for a full day on the day of purchase and the day of cancellation. 