I teach RE. Whilst it is your right to remove your child from RE, do not expect your child to receive any extra lessons in its place. The school does not have to provide this.
In the schools I have taught in, When students have been withdrawn from RE (usually JWs), they have gone to the library to do private study. Whether they actually do it, is usually another matter.
Do be aware, that if you do withdraw, this only applies for the year, and the head can call you in each year to meet and discuss whether your child is still to be withdrawn.
This is the govt guidance:
"Where parents have requested that their child is withdrawn, their right must be respected, and where RE is integrated in the curriculum, the school will need to discuss the arrangements with the parents or carers to explore how the child’s withdrawal can be best accommodated. If pupils are withdrawn from RE, schools have a duty to supervise them, though not to provide additional teaching or to incur extra cost. Pupils will usually remain on school premises."
However, I would encourage you to really have a look at your syllabus at secondary. Some of them are great. I'm not in secondary any more (now FE), but when I taught secondary we looked at atheism, agnosticism, humanism, all religions, cults and sects as well as the traditional religions. At KS4, we did philosophy and ethics, common to most schools.