Actually I am not really convinced that the introduction of new CPU lines has a dramatic effect on existing CPU pricing. If you look at Haswell pricing over the last 18 months you will see the average price is relatively stable after first introduction. That is, after the price drops from when it is first brought to market CPU pricing remains relatively stable. In this 18 month time frame Intel has introduced Skylake and Kaby Lake.
https://ca.pcpartpicker.com/trends/price/cpu/
As to whether to put money into your aging LGA 1155 system, that is the question. There is/will be nothing in the 160-180 GBP range for new Skylake/Kaby Lake or even Ryzen. The i5 3550 is still a valid CPU although starting to lag behind Sky Lake / Kaby Lake i5's. But still faster than an i3-7100.
http://www.cpubenchmark.net/compare.php?cmp%5B%5D=825&cmp%5B%5D=2910&cmp%5B%5D=2924
A 3550 with a RX 470 should have no problem playing Arma at 1080p. A new i5-7500 system can be had for about 300 GBP and would give you better performance but how noticeable it would be is a question. Both the 3550 and the 7500 are tier 1 CPU's in Tom's CPU hierarchy chart. Choice is up to you.
http://www.tomshardware.com/reviews/cpu-hierarchy,4312.html
PCPartPicker part list:
https://uk.pcpartpicker.com/list/YVgwjc
Price breakdown by merchant: https://uk.pcpartpicker.com/list/YVgwjc/by_merchant/
CPU: Intel Core i5-7500 3.4GHz Quad-Core Processor (£188.00 @ Amazon UK)
Motherboard: MSI B150M PRO-VDH Micro ATX LGA1151 Motherboard (£63.99 @ Ebuyer)
Memory: ADATA XPG Z1 8GB (2 x 4GB) DDR4-2133 Memory (£52.36 @ Amazon UK)
Total: £304.35
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-02-02 16:27 GMT+0000