Many thanks for your patience. The prohibitions on requiring employees to work on Sundays only applies to shop workers. So you will only be protected if you are working in a shop. What the law says is that whilst the employer can ask you to work on a Sunday, you have the right to opt out of that by giving them notice advising them you do not wish to do so and once that notice is given, you should not be asked to work Sundays after 3 months have elapsed.
In terms of flexible working, you would be making an application asking the employer for specific changes. So you are not just asking for flexible working without nominating what changes you are actually after and leaving the decision squarely with the employer. So it is not a question of applying for flexible hours and then be given changes that the employer thinks are required, when you in fact did not want these. You will be asking for the changes you are after and the employer would either accept these, or reject them and leave your terms as they are now, or make new suggestions. They should not just force changes on you which you have not asked for or which you do not want.
If you do come to an agreement on what changes you want then your pay can be reduced accordingly, for example if you are going from 5 days to 3 days then your pay can be reduced to reflect the fewer days you are working. The same applies if you are going to have to be placed into a different position to be able to accommodate these changes. But remember that they should only be made if you have asked for them and are happy to accept them, this is not a situation where the employer just makes the changes without consulting with you.
I hope this has answered your query. I would be grateful if you could please take a second to leave a positive rating (3, 4 or 5 stars) as that is an important part of our process and recognises the time I have spent assisting you. If you need me to clarify anything before you go - please get back to me on here and I will assist further as best as I can. Thank you