From the UK CAA website.
JAR Class 1 (Professional Pilot) Initial Examination Visual Standards
These are the JAR Class 1 visual standards as applied by the UK CAA.
Distance Vision
Your visual acuity (measured by your ability to see, in this case, lines of letters on a chart at 6 metres) must be at least 6/9 in each eye separately and 6/6 using both eyes together, with or without glasses or contact lenses (correction). If you need correction the refractive error (the amount of correction) must not exceed +5.00 dioptres of long sight or -6.00 dioptres of short sight. This is in the most ametropic meridian (taking into account any astigmatism). Astigmatism must not exceed 2.00 dioptres. The difference in correction between each eye (anisometropia) must not be more than 2.00 dioptres. Your optometrist will be able to explain these terms.
However, the renewal limits are significantly higher and with astigmatism and anisometropia above 3 dioptres class one holders require a comprehensive eye examination every 2 years.
You got through the intial which with eye sight is a lot stricter. By the sound of it your optician is confident that your astigmatism will not get worse. Actually your astigmatism is below the level that requires a comprehensive eye examination so I wouldn't worry about it too much. Talk to an AME or an optician but I would suspect that with regular eye check ups etc that you will be fine.
Visual - JAR Class 1 Comprehensive Eye Examinations | Medical | Safety Regulation contains all the renewal info
S