Insurer, and Vehicle Excise Duty (VED) / licence data, are shared by the relevant authorities including the Police and this forms an integral part of the mechanism of CIE. All UK registered vehicles, including those that are exempt from VED (for example, Historic Vehicles and cars with low or zero emissions) are subject to the VED taxation application process. Part of this is a check on the vehicle's insurance. A physical receipt for the payment of VED was issued by way of a paper disc which, prior to 1 October 2014, meant that all motorists in the UK were required to prominently display the tax disc on their vehicle when it was kept or driven on public roads. This helped to ensure that most people had adequate insurance on their vehicles because insurance cover was required to purchase a disc, although the insurance must merely have been valid at the time of purchase and not necessarily for the life of the tax disc. To address the problems that arise where a vehicle's insurance was subsequently cancelled but the tax disc remained in force and displayed on the vehicle and the vehicle then used without insurance, the CIE regulations are now able to be applied as the Driver & Vehicle Licence Authority (DVLA) and the MID databases are shared in real-time meaning that a taxed but uninsured vehicle is easily detectable by both authorities and Traffic Police. From 1 October 2014, it is no longer a legal requirement to display a vehicle excise licence (tax disc) on a vehicle. This has come about because the whole VED process can now be administered electronically and alongside the MID, doing away with the expense, to the UK Government, of issuing paper discs.
Yes, you surely can avail the NCB if you change your insurance provider at the time of renewing the policy. All you would need to do is producing a proof of the earned NCB from your current insurance provider. You can produce the original copy of your expiring policy and a certification that you haven’t filed any claim for the (expiring) insurance plan. A renewal notice or a letter stating that you’re entitled to the NCB from your previous insurance provider can be a proof for this.
Some classes of vehicle ownership, or use, are "Crown Exempt" from the requirement to be covered under the Act including vehicles owned or operated by certain councils and local authorities, national park authorities, education authorities, police authorities, fire authorities, health service bodies, the security services and vehicles used to or from Shipping Salvage purposes. Although exempt from the requirement to insure, this provides no immunity against claims being made against them, so an otherwise Crown Exempt authority may choose to insure conventionally, preferring to incur the known expense of insurance premiums rather than accept the open-ended exposure of effectively, self-insuring under Crown Exemption.
But liability coverage levels come in threes — you’ll probably see something like 50/100/50 up to 250/500/250 in typical policies. You can think of these limits like: individual injuries / total injuries / property damage. Insurers are a little more technical, calling them bodily injury liability, total bodily injury liability and physical damage liability.