The difficulty with registration such as yours is that if your car is burned beyond recognition, stolen, or otherwise unavailable for inspection by your insurer after a loss, the value will be largely based on what you have it registered as: in your case a 45-year-old VW bug.
All is not lost, however. Buy a policy that has an "agreed value" component. The actual verbage may be slightly different in the UK, but the principle will be the same, i.e, you and your insurer both "agree" on the value of the car before any money changes hands. If your car is totalled, that agreed value is the amount they will pay, with no deductibles, depreciation, etc. The higher value of an agreed value policy also helps with collision damage, since a higher value for the car means that the insurer will pay more for repairs before totalling the car.
The value on an agreed value policy will likely be significantly higher than what the insurer will pay on a stated value policy. Shop around and talk to car guys/clubs in your area. They will know who the best companies are to deal with in your locale. Best of luck and let others know what you find out. We have a couple of Brit car nuts on this site.