I did this years ago with 4500hrs rotary. It was a doddle.
Firstly airline reaction. Some understood that helo time was valuable but there are some real ignorant people out there who think rotary time is worthless. Sad because it is very far from that tho with only 300hrs its hardly relevant. Don't expect any more than a polite passing interest from a future employer.
On the UK system the conversion required the requisite number of P1 aeroplane hours to be achieved so just burning avgas pointlessly for 80 or so hours and in your case, yes, a PPL (A) to achieve that.
You'll also need the tech exams which you didn't cover in rotary. It's certain these will include P of F (aeroplanes), VP propellors and pressurisation. It may require piston engines and supercharging depending on the initial type you convert to. You're likely to have the rest. (Hydraulics, AC electrics) so the burden of study is not great. These are all pretty simple subjects.
You'll need to pass the skills test so a period with a flying school to get you up to speed on that will be needed. Presumably you did some f/w in the initial helo course so its boning up to exam standard that's required. How long that particular piece of string becomes is up to your skill in learning to fly a plank accurately which is harder than you might imagine. You should be exempt the nav section of the test and possibly some of the night flying requirements but your CAA will tell you that when you ask for an assessment.
Then there's the IR which being fresh from a helo IR won't be a major deal but by the same token don't underestimate it. Even a kludge like a Seneca is a remarkably sensitive instrument platform and will be a new type to you at the same time which adds interest.
Best of luck.
For a career aeroplanes are the way to go, helos are just for fun these days unless you strike extremely lucky. Sadly the world is awash with (mainly ex military) helo pilots and there are nowhere near enough jobs to go around.