What would be the advantage of attaching a tent to your Volvo? Roof and awning type tents are more expensive than normal tents and more of a faff to setup. Decent vehicle camping setups like Oztent/Foxwing are overkill for UK use and are megabucks.
If you want cheap just get a decent tarp and attach it to roof-rack/guttering with bungees and put table and chairs under it during day and a popup tent under it at night for sleeping in. You can also get popup utility/toilet tents which give standing room for changing/showering in.