Zen's IPv6 implementation will give you a static /64 and a static /48. The /64 is used solely for the WAN interface of your router. The /48 is for delegation to your LANs - this gives you 2^16=65536 /64s. A /64 is the smallest prefix allocation normally used on IPv6 other than for point to point links, not least because it's the smallest allocation over which SLAAC works. SLAAC is way that many IPv6 devices acquire their address(es).
A /48 is a large allocation, but not unusual for IPv6. That said, I think there are very few Zen ADSL and FTTx customers whose would find a /56 limiting - that's 2^8=256 /64s. There is no need to scrimp over IPv6 addresses at this stage, and certainly not adopt the practices of some ISPs in the US, which only give a dynamic /60 or, worse still, a dynamic /64, causing all manner of implementation problems. However, even with the somewhat wasteful way I use IPv6, I'm only using five /60s from the sixteen available in the lowest /56 in my /48 allocation. I've deliberately restricted my usage to that lowest /56, so that the remaining 255 /56s could be handed back to Zen in the future without me having to reconfigure anything. I'd encourage anyone else implementing IPv6 on their networks to stick to a similar arrangement, rather than spreading their usage throughout their allocation.
It is best to think of the IPv4 Internet and IPv6 Internet as entirely separate. Translation mechanisms exist to allow some IPv4 endpoints to initiate a connection to an IPv6 endpoint, but many IPv4 endpoints have no way of accessing an IPv6 endpoint. An IPv6 endpoint wishing to initiate a connection to an IPv4 endpoint needs a NAT64 - and I don't believe Zen runs a customer NAT64.
I would encourage you to enable IPv6 if you have a suitable router, then run your devices "dual stack", with both an IPv4 and IPv6 address. The time will come when there are endpoints on the Internet with no IPv4 connectivity that you wish to access, and the best way to prepare for that is for your devices to have native IPv6 enabled. To prepare for endpoints wishing to connect to your network that are IPv6 only, it is best to run any services, such as mail or web servers, on both IPv4 and IPv6.
If NAT with port translation is inadequate for what you want to do over IPv4, your only option is to request a regrade from your single IPv4 address to a /29 (8 IPv4 addresses). I believe Zen have now removed the option of a free /29 at sign up: all new accounts get a single static IPv4 address unless they specifically request and order another option at the prevailing price. I suspect a lot of the people who ordered a /29 when it just required selecting a free-of-charge option at sign up did so because it seemed like a good idea, but have only ever used the gateway address, wasting the rest of the block. Consumer routers typically make the use of anything other than a single IPv4 address on the WAN side difficult or impossible, especially if you want the router to provide NAT for all devices that lack a dedicated IPv4 address.
Now that we've reached IPv4 exhaustion, Zen will be wanting to conserve IPv4 addresses so that they can continue to sign up new customers with a single static IPv4 address. I guess a £30 fee for a /29 is seen as sufficient to discourage those who do not really need a /29. I suspect this fee might also apply to new customers wanting a /29.
For good reasons, I had to move from the free /29 I got at sign up many years ago to a /28 (16 IPv4 addresses) many years ago. Allocation of a /28 or larger block required a RIPE form and a fee some way over the £30 you say Zen are now charging for a /29.
The lowest address of an IPv4 subnet is the network address, and the highest address is the broadcast address. Strictly speaking, neither of these addresses can be used, though it there is normally no barrier to using the network addresses of an allocation smaller than a /24 with 1:1 NAT so long as the address isn't x.y.z.0 or x.y.z.255. If possible, it's best to leave the network and broadcast addresses unused.
Zen use the highest address of your IPv4 block other than the broadcast address as your gateway address - this will be the address allocated to your router via IPCP. It is important not to configure a static IPv4 address in the PPPoA or PPPoE settings of your router, as Zen use a dynamic gateway address. As with a single IPv4 address, Zen will always allocate the same address to your router via IPCP. Every router I've come across allows you to use this gateway address for NAT, just as you do with a single IPv4 address.
Once you've allowed for the network address, subnet address and gateway address (which you can almost certainly use for NAT), you have five addresses remaining from a /29. These can be allocated in any way you choose. Exactly how you use them depends on your router and the arrangement of your internal network(s).