Once you have used your 10 KIT days, you will lose one week’s SMP for each week or part week that you work ( over the ten days).
You can request that you return to work earlier than your agreed day, and this will ensure you are not in any breach or you will need to return to work on your agreed date and not work any days before this date. You wouldn’t be able to work one day a week for your employer even if the SMP isn’t being paid.
However, you could negotiate a gradual return to work where you went back on reduced hours and built up to full-time hours. However, this could have an impact on your pay. One way around this, if you plan to return to work full time, is to use a mix of holidays to make up the rest.