Short answers, pulled from the story.
The Uniform Penny Post was a British postal reform introduced by the Post Office (Duties) Act 1840 (3 and 4 Vict. c. 96). It set a flat charge of one penny for delivering any letter anywhere in the United Kingdom of Great Britain and Ireland, regardless of distance, replacing a system where cost depended on distance and number of sheets.
Rowland Hill is credited with the proposal that led to the Uniform Penny Post. On the 13th of February 1837 he presented his plan to a government commission, then published it as Post Office Reform: its Importance and Practicability. His 1838 parliamentary proposal called for a uniform rate of one penny per half ounce for letters delivered between any post-towns in the British Isles.
In 1840, the first year of the Uniform Penny Post, the number of letters sent in the United Kingdom more than doubled. Within ten years of its introduction, the total had doubled again.
One penny in 1840, assessed against mean income, is equivalent to as much as £4.89 in 2013 values. For an established semi-skilled worker in 1840, sending a letter under the new rate represented a cost roughly equivalent to £1 in 2013 terms.
Robert Wallace initiated the campaign for cheap postage in 1835, arguing before a government commission that greater use of the postal system would increase government revenue. Richard Cobden and John Ramsey McCulloch also attacked the existing system from a free-trade perspective; McCulloch stated in 1833 that nothing contributed more to facilitating commerce than the safe, speedy, and cheap conveyance of letters.
No. Prepayment remained optional for a number of years after the Uniform Penny Post was introduced in 1840. Rowland Hill, even after becoming Secretary to the Post Office, continued to press for compulsory prepayment, and had originally proposed adhesive postage stamps and pre-printed envelopes as mechanisms to achieve it.