Your Battersea example: no prob in EC 135 P2
don't know Battersea?
Say EOW 1850kg (IFR but no floats), 90 mins fuel at 3kg/min = 270 kg, say 20% reserve = 40 mins x 3 kg = 120 kg, say 5 pax x 90kg = 450 kg. Total weight 2690kg, max Group A TOW 2835 kg up to more than 30°C, means much more possible payload than on a AS 355.
Also much more bagagge room on EC 135 than on AS 355 and the EC 135 is in my view the more modern and innovative a/c. Full Cat "A", including automatic enhanced rotorspeed by "Cat A" switch and better OEI performance.
Engines without cooling time, can be immediately stopped after touch down. Maintenance costs are much lower on the EC 135. First date at 400h and many components "on condition", doesnt' have 50h or 100h checks. With the "fenestron" lower ground risk than the conventional tailrotor on the AS.
The EC 135 seems to be more expensive in buying, but i don't know actual AS 355N prices.
But just my view!!!