From what I can gather, It's okay to send within the UK but to send overseas via parcelforce, these seem to be the rules:
Arms and ammunition
Carriage of firearms between registered firearms dealers
A service for Section 1 and 2 firearms and low-powered airguns (as defined by the Firearms Act 1968, as amended), imitation firearms, de-activated guns, antique firearms and paintball guns is available to contract customers only, at the absolute discretion of Parcelforce Worldwide, and is subject to Parcelforce Worldwide providing prior written agreement in advance. Where Parcelforce Worldwide provide concurrence to sending firearms, it is subject to a number of conditions which include:
• Collections and deliveries of section 1 and 2 firearms will only be accepted where it can be shown to Parcelforce Worldwide’s satisfaction that both the sender and the recipient are Registered Firearms Dealers. Section 1 and Section 2 firearms, low-powered airguns, imitation firearms, antique firearms and paintball guns may be permitted for zone 1 to 1 on express9, 10, AM, 24, 48 services; deliveries to zones 2 and 3 must be on an express48 service and include ‘do not air uplift’ sticker on the parcel.
• Parcelforce Worldwide reserve the right that if any item is item is sent on the incorrect service, or does not include the correct labelling, or is a prohibited / restricted item then a charge of £20 will be levied in addition to the carriage charge and returned to the sending customer.
• All firearms must be clearly labelled ‘Strictly addressee only’ and ‘Do not leave with a neighbour’.
Arms and ammunition
Carriage of firearms between registered firearms dealers
A service for Section 1 and 2 firearms and low-powered airguns (as defined by the Firearms Act 1968, as amended), imitation firearms, de-activated guns, antique firearms and paintball guns is available to contract customers only, at the absolute discretion of Parcelforce Worldwide, and is subject to Parcelforce Worldwide providing prior written agreement in advance. Where Parcelforce Worldwide provide concurrence to sending firearms, it is subject to a number of conditions which include:
• Collections and deliveries of section 1 and 2 firearms will only be accepted where it can be shown to Parcelforce Worldwide’s satisfaction that both the sender and the recipient are Registered Firearms Dealers. Section 1 and Section 2 firearms, low-powered airguns, imitation firearms, antique firearms and paintball guns may be permitted for zone 1 to 1 on express9, 10, AM, 24, 48 services; deliveries to zones 2 and 3 must be on an express48 service and include ‘do not air uplift’ sticker on the parcel.
• Parcelforce Worldwide reserve the right that if any item is item is sent on the incorrect service, or does not include the correct labelling, or is a prohibited / restricted item then a charge of £20 will be levied in addition to the carriage charge and returned to the sending customer.
• All firearms must be clearly labelled ‘Strictly addressee only’ and ‘Do not leave with a neighbour’.