Ever wonder why when you see price tags of items on sale, sometimes it is represented as a monetary amount and other times as a %? Some of this is just preference of the retailer, but there is a trick that helps you and works in most cases (always test!).
Lets say you have an item that is £2 and you decrease the price to £1. In this situation, saying "50% off" is clearly stronger than saying "£1 off". Studies have revealed that this is the case until the monetary amount exceeds £100. So on the other extreme, if you have an item that is £1000 and you are selling it for £800, it is better to cite "£200 off" rather than "20% off".
But, what happens if you are not selling something at a discount, but rolling out a price increase? In this case, as you'd probably expect, the opposite is usually true. So, lets assume your customer has a monthly subscription that is £500/mo. If you are increasing the price by less than £100 (to £599.99 or less), then you should quote the £ increase. If you are increasing the price by more than £100 (to £600 or more), then you should quote the % increase. Of course, sometimes you can get away without quoting the increase at all, but this depends on the situation.
As mentioned before, these are rules you should test, not take as a given. As with all pricing psychology, there will be variances depending on the product, location, culture, etc.