If you reduce your hours to work flexibly, you should expect to see a corresponding reduction in your pay and benefits. Speak to your employer before you make your request, to make sure you understand the impact of the proposed change on your pay and benefits (including your pension), and make sure you can still manage, especially given other fixed costs such as any monthly train fares or childcare costs that you may not be able to adjust in the same way.
If you become a part-time worker, you will have the right not to be less favourably treated than a full-time worker doing the same job as you (and not to treated worse than you were when you did the job full-time). Any pro-rata reductions or attempts to deny part-time workers benefits should be carefully examined. Differences are allowed, but only if they can be objectively justified. Part-time workers are also protected under equal pay and National Minimum Wage law.
If you drop down to part-time hours after a period spent on maternity leave, remember that any statutory annual leave that you built up during maternity leave must be based on the hours you were working before you went on leave, not your new part-time hours.