I think that to answer this question, you need to first ask yourself 'why do I want an IR?'. If the answer is to take yourself as cheaply as possible to the point at which you might get called to interview by an offshore operator, then the single engine Bristows option might something to consider. It gets your face known (to at least Bristows) and if and when you go offshore, your company will convert your IR to a multi engine as a part of the type rating course that you do on your assigned aircraft type.
Going straight to a multi engine IR, while more expensive, does however offer you a useful qualification if you dont want to go offshore or if you are not sucessful in your application. More and more onshore operators (police, air ambulance, corporate) and being persuaded of the safety advantages of using instrument rated pilots and an EC155, AS355 IR can be a valuable thing to have in your back pocket.
Like most things, your IR comes down to how risk adverse you are and how much the 'insurance' against not getting an offshore job is worth to you. Helimutt is spot on in saying that a SEIR in the UK is like a 'chocolate fireguard'. It is an IR in all respects other than you cannot use it!!
Can I also suggest that you look closely at the quality of the training provider (including their pass rates) as even 2 hours of remedial training and an IRT resit in the UK will set you back £4K plus.
Good luck in whatever route you choose