Depends what spec the raft is and when it was last serviced.
If it is a top spec ocean raft with all the food and water in it and it's all out of date your looking at over £300
If it's a costal raft and it done a year 18 months ago it will be £150 ish
just been through this myself.... approx cost £95 + VAT for service then the additional bits inside that need replacing flares / water / sea sicktablets etc.... then it depends on the condition of the raft you may need some repairs to it. on average a basic service will set you back £200 - £250 from maybe Ronhale marine in portsmouth - ocean safety in southampton - mayday marine also in portsmouth.
I decided to ebay mine in the end got £250 for it, it was 2 years out of service. I bought a new seago offshore to replace it for what it would cost me to get the old one serviced /forums/images/graemlins/grin.gif but it depends on the spec of your old raft - most rafts tend to have a 12 year lifespan nowadays, so may be an idea to start thinking about a new one soon.
My Ocean Safety 6 man valise had its first 3 year service over the winter. Quotes from a no of places in Solent, including from Ocean Safety, were in region of £200 including replacement items, VAT & delivery costs. In the end had it serviced by Norwest Marine in Liverpool. Actual service cost was £65 + VAT. Total cost including replacement pack items and VAT was £125 ( I was in their area so delivered the raft to them myself, the charge for courier back was about £20, £145 in total)