Vehicle insurance for a Rental Motorhome.

First of all, UK vehicle insurance is expensive. That's just the way it is, whether one is a UK resident or not. The cost of insuring a motorhome over a 7-week period will inevitably be substantial. UK roads are very congested and rural roads generally narrow and winding. The chances of a large vehicle being damaged are higher than those of a car and this is reflected in the cost of insurance.




UK 'third party' insurance covers damage to people (including  passengers other than the driver in the insured vehicle), to other vehicles and/or to property if the incident is your fault. It is the legal minimum requirement for any motor vehicle driven on the road in the UK. Hire companies are not allowed to rent vehicles without third party insurance being in place and would be legally liable if they did so. 
For many decades the basic third party option was by far the cheapest vehicle insurance but it's now very expensive indeed and very little cheaper than comprehensive insurance, if it's cheaper at all.










UK 'fully-comprehensive' insurance of course includes the legally-required third party aspect but each and every insurance policy has its own exemptions (e.g. windscreens, tyres) and excesses. It is often possible to minimise those exemptions and excesses but doing so always (and obviously) raises the cost of the policy. 





There are many UK insurance providers, each of which offers different policies. Insurances offered by hire companies vary according to the provider used by that company. 
As you are not clear about what this particular hire company offers you do need to contact them directly and ask for further clarification....most especially the 'third party' aspect. Any reputable company should be able to provide you with more detail, including whether it is possible to pay a higher premium to cover exclusions such as damage caused by reversing.
If you remain unhappy with what this particular company offers perhaps it might be worth exploring other UK motorhome hire companies?  You might find better insurances or, given that there aren't too many such companies,  you might find they are all much the same. If the latter, at least you'll know you're not being ripped-off by one particular company.