Does anybody know how much it costs in the UK to get the front screen of an iPhone4 replaced as it had smashed?

You can look this up yourself:

http://www.apple.com/support/service/

"

Out-of-Warranty Service

If you own an iPhone that is ineligible for warranty service but is eligible for Out-of-Warranty (OOW) Service, Apple will service your iPhone for the Out-of-Warranty Service Fee listed below.

Please note Out-of-Warranty Repair Service fees from your carrier may be different than those listed below.

The Out-of-Warranty Service Fee includes a £ 7.44 shipping fee, which applies only if service is arranged through Apple and requires shipping. All fees are in British pound sterling and include VAT. iPhone Out-of-Warranty Service may also be available from your wireless carrier. Pricing may vary."

THis is the same sort of problem I have except my whole phone is smashed? Would they still replace it for this price?

yes