The returns policy sets out discretionary or extra-statutory rights. It does not cover statutory rights to return products. For example, it does not cover the special 14-day cooling off period that benefits consumers under the distance selling laws. (Although the terms and conditions of sale template published on this website does cover some statutory rights, including the distance selling rules.)
There are few good reasons for treating statutory and non-statutory rights separately.
First, in general it is not possible to abridge statutory rights. Specifying ways of exercising statutory rights will often amount to just such an abridgement. Accordingly, if the returns policy were extended to cover statutory and non-statutory rights, the specific returns instructions would to some extent need to be framed as requests or suggestions.
Second, while one of the main purposes of offering a non-statutory returns policy is to help with marketing efforts, businesses should not present basic statutory protections as being a special benefit of dealing with them. If you combine your policies on statutory and discretionary returns, you risk accidentally tripping over this prohibition.
Third, in my experience, a combined policy covering both complex statutory rights and broad-brush contractual rights (which usually depend to a significant degree on retail discretion) is very often a messy synthesis requiring high-level legal expertise to write and interpret.
There are situations where a combined policy is appropriate. For instance, if a discretionary policy is changing cooling-off periods but otherwise closely tracking the distance selling rights, then it makes sense to offer a combined policy.
If you would like professional advice on creating your returns policy, please do get in touch.