Vendor/Module/etc/frontend/events.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="controller_action_predispatch">
        <observer name="change_title_filter" instance="VendorModuleObserverCustomercurrency" />
    </event>
</config>

Vendor/Module/Observer/Customercurrency.php

<?php

namespace VendorModuleObserver;

use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkEventObserver;
use MagentoFrameworkViewLayoutInterface;

class Customercurrency implements ObserverInterface
{
    protected $context;
    public function __construct(
        MagentoCustomerModelSession $customerSession,
        MagentoFrameworkViewElementTemplate $context,
        MagentoFrameworkViewResultPage $resultPage,
        MagentoFrameworkViewPageConfig $pageConfig,
        MagentoStoreModelStoreManagerInterface $storeManager

    ) {
        $this->_customerSession = $customerSession;
        $this->pageConfig = $pageConfig;
        $this->resultPageFactory = $resultPage;
        $this->context = $context;
        $this->_storeManager = $storeManager;
    }  
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        if($this->_customerSession->isLoggedIn()) {
            $customercurr = $this->helperData->getCurrenyCustomer();
            $customerGroup = $this->_customerSession->getCustomer()->getGroupId();
            $customergroupid = 1;
            if($customerGroup == $customergroupid){
                $this->_storeManager->getStore()->setCurrentCurrencyCode('GBP');
                break;
            }else{
                $this->_storeManager->getStore()->setCurrentCurrencyCode('USD');
            }            
        }
    }
}