I've got a /28 on my Zen connection. Any ISP will require the RIPE form for a /28 allocation or larger - you have to demonstrate the need for the addresses. You may be able to get a /29 without the form.
With IPv4 address exhaustion, many ISPs have withdrawn routed IPv4 products from open advertisement. At one point it was an option on sign up with Zen to have a single IP address or a /29 - as there was no cost difference, I suspect many people chose /29 and only ever used one address.
I'm pretty sure Zen will still offer a /28 subject to a satisfactory RIPE form and a one-off payment of £50+VAT for their administrative costs. You then pay the
regular price for the broadband product of your choice. I'd
contact Zen sales and ask if they can offer 16 IP addresses on a new ADSL account.
All Zen routed IP accounts use the highest usable IP address as the gateway address. Zen do not currently offer IPv6 on their broadband products - it's coming, but there's no timescale and apparently nothing is imminent.
I'm very happy with Zen - and have no links other than being a satisfied customer.