Letters up to 100g: 1st class up 2p to 67p, 2nd class up 2p to 58p.
Large letters up to 100g: 1st class up 3p to £1.01, 2nd class up 3p to 79p.

Small parcels up to & including 1kg: 1st class up 5p to £3.45, 2nd class up 5p to £2.95.
Small parcels up to & including 2kg: 1st class unchanged at £5.50, 2nd class up 5p to £2.95.

Medium parcels up to & including 1kg: 1st class up 5p to £5.75; 2nd class up 5p to £5.05
Medium parcels up to & including 2kg: 1st class unchanged at £8.95, 2nd class up 5p to £5.05.

Letters or postcards to anywhere in the world, up to 10g, go up 8p to £1.25 because of an increase in overseas delivery costs.

These are the main changes for the most common sizes. Full details of all new rates, including for franking machines, are at https://www.royalmail.com/prices2018. 

Generic stamps with 1st or 2nd on them rather than a money amount remain valid for that class of postage, regardless of when they were purchased – so there's nothing to stop you from stocking up this week.

If you are sceptical, see
https://blog.moneysavingexpert.com/2012/03/30/why-stamps-that-say-1st-stay-valid-in-perpetuity-so-hoard-em/
This article is from 2012, but the law hasn't changed.

source sandy adirondack 20.03.18