personally I like the look of...
5% every year rounded up to the next 10p*, I prefer enforce in Oct but thats just my inexperience maybe.
*eg, 5.00 and 5% = 5.25 round up to 5.30
5.30 and 5% = 5.56 round up to 5.60
in this case gives a 12% increase in two years and does not feel that bad to customer.
5.60 and 5% = 5.88 round up to 5.90
5.90 and 5% = 6.20
and so on. I know window cleaners like to move in 50p increments but I think it is a bit much for the lower, sometimes bread and butter, end of the market
PS, in relation to the actual question.
Put it up to £10 because next year you will want it at £10.50 just to stop you doing it for free. At the very least tell the customer that you should be putting it up to £10 and feel that £9 this year and £10.00 next year will be easier for her/him
If they drop you, you can come and work for me for £7.50 an hour

7.50 and 5% = 7.87 (7.90)
7.90 and 5% = 8.30
8.30 and 5% = 8.72 (8.80)
8.80 and 5% = 9.24 (9.30)
9.30 and 5% = 9.76 (9.80)
which is not far off the £10 you want.