It all depends on the kayak, its construction, and its design. In the US, some to consider for that purpose are Tributary Sawtooth, and Advanced Elements makes a few models suitable for mild coastal use. In the UK, Innova has some models well suited to coastal water. I've got an Innova whitewater kayak, but I'm too big to use it. You're not, but it's pretty short for coastal use so I'm not sure that it makes sense to work out a deal. There are many more reputable manufacturers of inflatable kayaks. I suggested these as something for you to Google and check out.
In general, a longer boat means you'll float shallower and may get better "tracking." Tracking is how the boat tends to go in a straight line when paddled. Depending on the water you're on, this can be a good thing, a bad thing, or just a thing. For example, the 10' kayak you borrowed from Dirty Ed on the Shenandoah was just about perfect for that water. It turned easily, the current carried it but you could correct easily. But it wouldn't be very suitable if you had to cross a wide lake or an expanse of flat water, because it's too short to track real well and you'd be more at the mercy of the wind than you'd be with a longer keel-line. I have a 14' skinny kayak that tracks straight as an arrow and is very fast for that kind of long flat-water crossing, but it's a bear turning it in time to avoid rocks on a shallow river. My recommendation based on your intended use, my knowledge of your size, and what to expect from an inflatable kayak is to consider 3.5 meters length as a baseline and adjust from there based on availability and personal preference.
Some things to consider are whether it is a sit-in style (like the Advanced Elements AdvancedFrame) or a sit-on style without an upper deck (like the Tributary Sawtooth). Given that you are inexperienced as a kayaker, I think a sit-on would be safer for you to use because it is much easier to recover from being swamped. That said, the sit-ins are nice because they deflect a lot of water, especially if you wear a skirt while paddling. A kayak skirt. Not one from Dress Barn.
Pros and cons of inflatables versus hard shell kayaks:
Inflatables are easier to transport.
Inflatables are easier to store.
Inflatables are harder to control in wind (they are essentially very firm balloons and tend to have a higher profile off the water).
Hard shells are easier to set up.
Hard shells are quicker to launch.
Overall, it comes down to what fits your lifestyle and what suits your fancy. I love how a well made inflatable in nearly indestructible. I've confidently taken inflatable whitewater kayaks down rivers I'd be hesitant to take a hard shell kayak.
One site that has a huge amount of reliable information on inflatable kayaks is theboatpeople.com. I have no relationship to them other than that of a satisfied customer. They are extremely helpful via e-mail. I don't know if they can ship to the UK, but here in the US they consistently carry high quality products at fair prices.
There are a lot of inexpensive inflatable kayaks out there. They're pool toys, not real boats.
Lastly, always, under every circumstance, in every instance, wear a PFD from dry land to dry land. Put it on on the bank and do not remove it until you are back on the bank. Spend a little extra to get one that is comfortable and that you will wear.
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