If you have an Openreach FTTP connection then move to VM with your eyes open. When it works, the VM service is fast and reasonably reliable, but it relies on analogue technology (DOCSIS), and in most areas that is across ageing coaxial networks, although EX5 is presumably Cranbrook, and may well be VM's newer RFoG networks that are FTTP. Latency (ping) will often be be more erratic than an FTTP connection - when all is working well that won't be noticeable, if things aren't perfect it will be noticeable. If faults arise, you'll have to deal with VM's legendarily poor customer service, delivered through some very ropey "chatbots" and the sort of poor quality offshore outsourcing of the type that gets offshoring and outsourcing a bad name. This forum and its staff are a tiny beacon of light and civilisation in VM's universe of darkness and disorder, and whilst it can't work miracles, the UK based staff do their best to help customers get things fixed. A current problem is that some (not all) new installations are in utter chaos, with repeated promises to connect customers being broken, and the actual connection taking many months. VM's email services have had repeated faults and issues, so bear that in mind if planning to use that. Customer communication can at times be unbelievably poor, with no apparent connection between the teams who "do stuff" and those involved in speaking to customers. You mention worries about congestion. These really shouldn't be a problem in a new build area with a modern VM network, although if you did encounter them then VM's handling of these is truly dire.
When major or widespread problems occur, VM's immediate corporate response is one of silence or even denial, which makes for very frustrated customers. An example earlier this year was the network capacity problems caused by lockdown and home working. VM were not alone in suffering those, although the DOCSIS technology appeared to suffer more than Openreach, but VM's management refused to acknowledge or explain the problem, real heads in the sand (or somewhere else). This led to a massive spike in complaints, and VM's main acknowledgement of this has been to congratulate itself for the subsequent tailing off of complaints as "improvements in customer service".
Browse through the forum sections and see what complaints come up (particularly long running threads), and also checkout Trustpilot and ISP Review for customer experiences, although by definition a help forum is full of problems, and customers are motivated more to leave poor reviews than good ones, ISPReview is perhaps the best comparative sample because it's something of a (very good) niche site with many technically aware readers.
You say you want to move to VM to save money. Fair enough, but VM (certainly ignoring O2) are structurally loss making, so prices need to go up considerably in the longer term. Introductory prices can be competitive, but they go up hugely at the end of the contract, and whilst some customers negotiate better deals, VM, unlike most other ISPs, have refused to make new customer pricing available to existing customers through negotiation. When you join VM you should expect inflation busting price rises each year (even during your 18 month minimum commitment) which is probably no different to BT. Have you either tried to negotiate a discount with BT's customer retention team (phone them up, say you're about to take the VM deal at £x a month, that'll save you £yyy over the 18 month term, can't they match that?), or seek out other ISPs using Openreach infrastructure, who you could join as a new customer?
If you decide you do want to try VM, don't cancel the BT connection before you have a working VM connection. Leave that in place, order VM, seen how that all works, and when connected compare the two side by side during your 14 day cooling off period, and only cancel BT if you're happy that the VM connection is equally good. You will pay for both connections during the overlap (five-six weeks including the BT notice period), but this avoids the risks of a delayed VM installation leaving you without any connection, and if the VM service is not to your satisfaction you can cancel that and stick with BT.
