If the homework is being assigned at the start of school, it may be that the assignments are more intended as assessments, rather than actual lessons at this point.
Particularly at age 4, it seems unreasonable to expect every child to be able to properly form letters and identify words starting with that letter without a significant amount of practice/studying, but not unreasonable to check to see how many kids can do this already.
Some children do pick up these skills by four, particularly if they have attended pre-school programs or have parents who may have spent time going over this with them prior to entering the mainstream school system.
Regarding your specific questions... I can't answer #1, as I have no experience with London's school systems, but I can speculate that your daughter may be singled out as being behind in performance compared to others, or, more likely, you will be singled out as being uncooperative. This may or may not be a problem overall, depending upon what you expect to get out of the system.
As for #2, I'd personally let it be for a while, allowing my child to do the work on their own as best they can, and seeing how the teacher(s) respond. If you really don't believe that learning to write is a priority at this time, that's your prerogative. If the current assignments are more intended as assessments than course-work, then you may see the homework be replaced with more agreeable materials fairly soon. If not, then you may have a situation where your daughter will be putting focus into areas you don't believe are appropriate.
In this case, the next step would be to contact the teacher(s) directly, and discuss your concerns. This may be somewhat less productive if you've established yourself as "uncooperative" already.
If you are unable to resolve it by discussing it with a teacher, then the options left to you are likely finding another school (either another district, or perhaps a private school, depending upon what is allowed/available), having your child conform to the teacher's expected workload, or (the least attractive, in my opinion) having your child ignore the assignments you disagree with, which risks not only creating further conflict with your teacher, but puts your child in the middle, and establishes a precedent that they can ignore any topic or assignment they don't like.