you should only put glue on the outside of a pipe. Putting it on the inside means itll get scraped up into a nice big blob ny the ingoing pipe, and stick proud of the joint inside. This provides a perfect place for solids in the wastewater to start collecting.. One hair, then two.. then some conditioner, then more hair, and before you know it the pipe is blocked solidMy local Screwfix doesn't stock the solvent cement (no idea why), so you're lucky there. Solvent weld is rock solid if done properly; put the solvent glue on both surfaces
Same goes for leaving rough edges if cutting with a hacksaw. Invest in a pipe deburrer, or get good at running a Stanley knife round the inner corner of the cut